ADR-0024 HintMode dispatch via walker_hint_mode_at_input

Adds the `HintMode` dispatch layer the ADR specified: the
ambient-hint resolver now consults a single
`walker::hint_mode_at_input(source) -> Option<HintMode>` to
decide between the prose / candidates ladder, rather than
discovering each slot kind through three separate post-hoc
helpers (`value_literal_hint_at_cursor`,
`typing_name_at_cursor`, and so on).

Behaviour at slot positions today:

- **Value-literal slot** (`null`/`true`/`false`/number/string
  all in the expected set) → `HintMode::ProseOnly
  ("hint.value_literal_slot")`. The ambient-hint ladder
  emits the catalog prose at empty prefix; once the user types
  a partial (`n`, `tr`, `fa`) the partial check declines and
  normal candidate completion takes over.
- **NewName ident slot** → `HintMode::ForceProse
  ("hint.ambient_typing_name")`. The ladder still consults
  `typing_name_at_cursor` to learn what comes after the name
  (the post-name probe is unchanged); `ForceProse` is the
  declarative tag telling the resolver *that* we're in this
  mode.

`HintMode` itself gains `PartialEq + Eq` for tests, and
its docstring is rewritten to describe the live semantics.

This is the structural shape ADR-0024 §HintMode-per-node
describes: one slot → one hint mode → one dispatch arm. The
detection inside `hint_mode_at_input` is transitional — it
pattern-matches the walker's expected-set today, which is
exactly what the previous ad-hoc detectors did. Phase D will
replace the signature match with node-attached `HintMode`
annotations on the typed value slots (so `date_slot`,
`int_slot`, etc. each carry a type-specific catalog key).

Two helpers move into `input_render.rs`:
- `hint_leading_slice(input, cursor)` mirrors the look-back
  used by `candidates_at_cursor` so the hint resolver sees the
  same token-boundary view of the world.
- `cursor_partial_is_empty(input, cursor)` distinguishes
  empty-prefix from in-progress identifier shapes.

8 new walker tests pin the hint-mode resolver across
value-literal-after-paren, value-literal-after-set-assign,
value-literal-in-where, two NewName-slot cases, the
entry-keyword position, the complete-command position, and
the schema-ident position.

Tests: 817 passing, 0 failing, 1 ignored. Clippy clean.

src/dsl/grammar/mod.rs

@@ -119,13 +119,25 @@ impl IdentSource {
     }
 }
 
-/// Hint-panel mode for an expected node.
+/// Hint-panel mode for an expected node (ADR-0024 §HintMode-per-node).
 ///
-/// Phase A defaults to `Default`; the `ProseOnly` variant
-/// attaches to typed value slots in Phase D so the hint reads
-/// "Type a date as 'YYYY-MM-DD'" rather than candidate-cycling.
-#[derive(Debug, Clone, Copy)]
-#[allow(dead_code)]
+/// `Default` (today's behaviour) shows candidates if any, falls
+/// back to a prose ladder otherwise. The other variants
+/// override at slot positions where the candidate list would be
+/// actively misleading or where the user benefits from format
+/// guidance:
+///
+/// - `ProseOnly(catalog_key)` — show only prose from the
+///   catalog; suppress Tab candidates. Used today by the
+///   value-literal slot at empty prefix (the "null/true/false"
+///   candidate trio is misleading at a slot that more often
+///   takes a number / quoted text / date).
+/// - `ForceProse(catalog_key)` — force this prose at the
+///   catalog key regardless of candidates. Used today by
+///   `NewName` ident slots ("Type a name, then `(`").
+/// - `SuppressProse` — show only candidates; never fall back
+///   to a prose ladder.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum HintMode {
     Default,
     ForceProse(&'static str),

src/dsl/walker/mod.rs

@@ -30,6 +30,73 @@ use crate::dsl::walker::outcome::{
 pub use context::ColumnInfo;
 pub use highlight::highlight_runs;
 
+/// Resolve the hint-panel mode at the end of `source` (ADR-0024
+/// §HintMode-per-node).
+///
+/// Today this is a detection-based bridge: the walker's
+/// expected-set is pattern-matched for the value-literal slot
+/// signature (`null`/`true`/`false`/number/string) and for
+/// `Ident { source: NewName }`. The mapping is exactly what the
+/// post-hoc ad-hoc cases in `input_render.rs::ambient_hint`
+/// used to compute inline — relocated to one place so the hint
+/// resolver dispatches on a `HintMode` enum rather than
+/// rediscovering the cases at every call site.
+///
+/// Phase D will replace this with node-attached `HintMode`
+/// annotations on the typed value slots (so `date_slot` carries
+/// `ProseOnly("hint.date_format")`, `int_slot` carries an
+/// integer-specific hint, etc.). The signature pattern-match
+/// here becomes obsolete once that lands.
+#[must_use]
+pub fn hint_mode_at_input(source: &str) -> Option<crate::dsl::grammar::HintMode> {
+    use crate::dsl::grammar::{HintMode, IdentSource};
+    use crate::dsl::walker::outcome::Expectation;
+
+    let expected = expected_at_input(source);
+    if expected.is_empty() {
+        return None;
+    }
+
+    // Value-literal slot signature: all five forms present.
+    let has_word = |w: &str| {
+        expected
+            .iter()
+            .any(|e| matches!(e, Expectation::Word(x) if *x == w))
+    };
+    let value_literal_slot = has_word("null")
+        && has_word("true")
+        && has_word("false")
+        && expected.iter().any(|e| matches!(e, Expectation::NumberLit))
+        && expected.iter().any(|e| matches!(e, Expectation::StringLit));
+    if value_literal_slot {
+        // The catalog wording lists all valid literal forms with
+        // format examples. Phase D will narrow per column type.
+        return Some(HintMode::ProseOnly("hint.value_literal_slot"));
+    }
+
+    // NewName ident slot: user invents a name.
+    let new_name_slot = expected.iter().any(|e| {
+        matches!(
+            e,
+            Expectation::Ident {
+                source: IdentSource::NewName,
+                ..
+            }
+        )
+    });
+    if new_name_slot {
+        // The "Type a name" prose key is selected by the
+        // ambient_hint dispatch — the `ForceProse` key here is
+        // a stable identifier the resolver maps to one of the
+        // two variants (`hint.ambient_typing_name` /
+        // `hint.ambient_typing_name_then`) depending on whether
+        // a next-token probe yields content.
+        return Some(HintMode::ForceProse("hint.ambient_typing_name"));
+    }
+
+    None
+}
+
 /// What the grammar would accept at the end of `source`
 /// (ADR-0024 §architecture, Phase F walker-driven completion).
 ///
@@ -1025,4 +1092,77 @@ mod tests {
             }
         );
     }
+
+    // =========================================================
+    // hint_mode_at_input (ADR-0024 §HintMode-per-node)
+    // =========================================================
+
+    use crate::dsl::grammar::HintMode;
+    use super::hint_mode_at_input;
+
+    #[test]
+    fn hint_mode_value_literal_slot_after_insert_open_paren() {
+        // `insert into T (` expects a value-literal or column
+        // ident at the inner position. After `values (` it's
+        // strictly value-literals — the signature triggers
+        // ProseOnly.
+        match hint_mode_at_input("insert into T values (") {
+            Some(HintMode::ProseOnly("hint.value_literal_slot")) => {}
+            other => panic!("expected ProseOnly value_literal_slot, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn hint_mode_value_literal_slot_after_update_set_assign() {
+        match hint_mode_at_input("update T set col=") {
+            Some(HintMode::ProseOnly("hint.value_literal_slot")) => {}
+            other => panic!("expected ProseOnly value_literal_slot, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn hint_mode_value_literal_slot_in_where_clause() {
+        match hint_mode_at_input("delete from T where col=") {
+            Some(HintMode::ProseOnly("hint.value_literal_slot")) => {}
+            other => panic!("expected ProseOnly value_literal_slot, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn hint_mode_new_name_slot_for_create_table() {
+        // `create table ` expects a NewName ident.
+        match hint_mode_at_input("create table ") {
+            Some(HintMode::ForceProse("hint.ambient_typing_name")) => {}
+            other => panic!("expected ForceProse typing_name, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn hint_mode_new_name_slot_for_add_column_name() {
+        // `add column T: ` expects a NewName ident.
+        match hint_mode_at_input("add column to table T: ") {
+            Some(HintMode::ForceProse("hint.ambient_typing_name")) => {}
+            other => panic!("expected ForceProse typing_name, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn hint_mode_none_for_keyword_position() {
+        // Entry-keyword position: no HintMode override applies.
+        assert!(hint_mode_at_input("").is_none());
+        assert!(hint_mode_at_input("cr").is_none());
+    }
+
+    #[test]
+    fn hint_mode_none_for_complete_command() {
+        // Valid complete command: no expected, no override.
+        assert!(hint_mode_at_input("create table T with pk").is_none());
+    }
+
+    #[test]
+    fn hint_mode_none_at_schema_ident_slot() {
+        // `show data ` expects a table-name ident from the
+        // schema — schema-listable slot, not a HintMode case.
+        assert!(hint_mode_at_input("show data ").is_none());
+    }
 }

src/input_render.rs

@@ -177,28 +177,38 @@ pub fn ambient_hint(
             selected: Some(m.selection_idx),
         });
     }
-    // No memo: fall back to candidates_at_cursor. When any
-    // exist the user can Tab to insert one, and the panel
-    // surfaces them directly — this wins over the prose
-    // IncompleteAtEof framing because the candidate list is
-    // more actionable.
-    if let Some(comp) = crate::completion::candidates_at_cursor(input, cursor, cache) {
-        return Some(AmbientHint::Candidates {
-            items: comp.candidates,
-            selected: None,
-        });
+    // Resolve the walker-side `HintMode` at the cursor. This
+    // detects value-literal-slot and NewName-slot positions
+    // declaratively (ADR-0024 §HintMode-per-node) — the
+    // ambient-hint ladder dispatches on the returned variant
+    // before falling through to the generic candidates / prose
+    // framings.
+    //
+    // We pass the `leading` slice (input up to the start of any
+    // partial identifier the user is mid-typing) so the hint
+    // mode reflects the slot expected at the token boundary,
+    // not whatever the partial would resolve to.
+    let leading = hint_leading_slice(input, cursor);
+    let hint_mode = crate::dsl::walker::hint_mode_at_input(leading);
+    match hint_mode {
+        Some(crate::dsl::grammar::HintMode::ProseOnly(key)) => {
+            // The cursor sits at a slot where Tab candidates
+            // would be actively misleading. Surface the catalog
+            // prose for the slot. Only fires at empty prefix —
+            // once the user starts typing a partial, normal
+            // candidate completion (e.g. `n` → `null`) applies.
+            if cursor_partial_is_empty(input, cursor) {
+                return Some(AmbientHint::Prose(crate::friendly::translate(key, &[])));
             }
-    // Value-literal slot at empty prefix: candidates_at_cursor
-    // returned None (it suppresses null/true/false here to avoid
-    // implying they're the only options). Surface a prose hint
-    // that names all valid literal forms with format examples.
-    if let Some(prose) = crate::completion::value_literal_hint_at_cursor(input, cursor) {
-        return Some(AmbientHint::Prose(prose));
         }
-    // User typing into a NewName slot — show the friendlier
-    // "type a name" hint rather than the technical "next: …"
-    // that the post-consumed-partial parse would otherwise
-    // produce (round-3 follow-up).
+        Some(crate::dsl::grammar::HintMode::ForceProse(_key)) => {
+            // NewName slot: show "Type a name [then <next>]".
+            // The probe in `typing_name_at_cursor` reads what
+            // would come *after* the name, so we still consult
+            // it to populate the optional `then` clause. The
+            // walker-side `ForceProse` annotation tells us
+            // *that* we're in this mode; the probe tells us
+            // *what comes next*.
             if let Some(t) = crate::completion::typing_name_at_cursor(input, cursor) {
                 let text = t.next_after_name.map_or_else(
                     || crate::t!("hint.ambient_typing_name"),
@@ -206,6 +216,21 @@ pub fn ambient_hint(
                 );
                 return Some(AmbientHint::Prose(text));
             }
+        }
+        Some(crate::dsl::grammar::HintMode::SuppressProse | crate::dsl::grammar::HintMode::Default)
+        | None => {}
+    }
+
+    // No HintMode override: candidate-or-prose ladder applies.
+    // Candidates win when any exist — the panel surfaces them
+    // directly because they're more actionable than prose
+    // framings.
+    if let Some(comp) = crate::completion::candidates_at_cursor(input, cursor, cache) {
+        return Some(AmbientHint::Candidates {
+            items: comp.candidates,
+            selected: None,
+        });
+    }
     // Invalid identifier: cursor sits in a known-set slot but
     // the typed prefix matches nothing in the schema. (Stage
     // 8e / the user's #5.)
@@ -265,6 +290,38 @@ pub fn ambient_hint(
     }
 }
 
+/// Slice of `input` ending at the start of the partial
+/// identifier-shape token at `cursor` (if any). Mirrors the
+/// look-back used by `completion::candidates_at_cursor` so the
+/// hint resolver sees the same "what was expected at the token
+/// boundary" view of the world.
+fn hint_leading_slice(input: &str, cursor: usize) -> &str {
+    let cursor = cursor.min(input.len());
+    let bytes = input.as_bytes();
+    let mut start = cursor;
+    while start > 0 {
+        let prev = bytes[start - 1];
+        if prev.is_ascii_alphanumeric() || prev == b'_' {
+            start -= 1;
+        } else {
+            break;
+        }
+    }
+    &input[..start]
+}
+
+/// True when the cursor is at a token boundary — no partial
+/// identifier-shape token in progress.
+fn cursor_partial_is_empty(input: &str, cursor: usize) -> bool {
+    let cursor = cursor.min(input.len());
+    let bytes = input.as_bytes();
+    if cursor == 0 {
+        return true;
+    }
+    let prev = bytes[cursor - 1];
+    !(prev.is_ascii_alphanumeric() || prev == b'_')
+}
+
 /// "A, B, or C" / "A or B" / "A". Local copy because the
 /// parser's identical helper is private.
 fn oxford_or(items: &[String]) -> String {