ui: surface diagnostics in the ambient hint panel (ADR-0027 §2)

ambient_hint now reads the walker's schema-aware diagnostics.
input_diagnostics is non-empty only for a command that
structurally parses — so a non-empty result means "complete
and submittable, but wrong or dubious". That is checked early
(right after the Tab-cycle memo), ahead of slot hints and
completions: a command that parses but is flawed no longer
gets the misleading "Submit with Enter" prose, it gets the
diagnostic's why. pick_hint_diagnostic prefers the diagnostic
under the cursor, else the most severe.

The cursor-local invalid-ident hint is kept for genuinely
incomplete commands (no Match → no diagnostics).

5 ambient_hint tests (unknown table, type-mismatch over
submit-prose, LIKE-numeric, clean command still submittable,
cursor-following). The complex_and_or matrix cell referenced a
non-existent column `t`; fixed to a real column so it tests a
valid expression as intended. 1118 passing, clippy clean.

src/input_render.rs

@@ -223,6 +223,21 @@ pub fn ambient_hint(
             selected: Some(m.selection_idx),
         });
     }
+    // Schema-aware diagnostics (ADR-0027 §2). `input_diagnostics`
+    // is non-empty only for a command that *structurally parses*
+    // — so a non-empty result means "this command is complete
+    // and submittable, but wrong or dubious". That is the single
+    // most important thing to tell the user, ahead of slot hints
+    // and completions, so it is checked early — right after the
+    // Tab-cycle memo. An unknown name is an ERROR, a dubious
+    // comparison a WARNING; the diagnostic under the cursor wins
+    // (the panel explains where the user is looking), else the
+    // most severe one. The error overlay still marks every
+    // flagged token; this panel carries the *why*.
+    let diagnostics = crate::dsl::walker::input_diagnostics(input, Some(cache));
+    if let Some(diag) = pick_hint_diagnostic(&diagnostics, cursor.min(input.len())) {
+        return Some(AmbientHint::Prose(diag.message.clone()));
+    }
     // Resolve the walker-side `HintMode` at the cursor. This
     // detects value-literal-slot and NewName-slot positions
     // declaratively (ADR-0024 §HintMode-per-node) — the
@@ -418,6 +433,26 @@ fn oxford_or(items: &[String]) -> String {
     }
 }
 
+/// Choose which diagnostic the hint panel surfaces: the one
+/// whose span contains the cursor (the panel explains where the
+/// user is looking), else the most severe — ERROR over WARNING,
+/// ties broken leftmost.
+fn pick_hint_diagnostic(
+    diagnostics: &[crate::dsl::walker::Diagnostic],
+    cursor: usize,
+) -> Option<&crate::dsl::walker::Diagnostic> {
+    diagnostics
+        .iter()
+        .find(|d| d.span.0 <= cursor && cursor <= d.span.1)
+        .or_else(|| {
+            diagnostics.iter().max_by(|a, b| {
+                a.severity
+                    .cmp(&b.severity)
+                    .then_with(|| b.span.0.cmp(&a.span.0))
+            })
+        })
+}
+
 fn overlay_error(runs: &mut [StyledRun], error_byte: usize, theme: &Theme) {
     // Failing tokens have their byte_range starting exactly at
     // `error_byte`. Override the fg colour while preserving any
@@ -958,7 +993,9 @@ mod tests {
     #[test]
     fn ambient_hint_for_invalid_identifier_says_no_such() {
         use crate::completion::SchemaCache;
-        // Schema knows "Customers"; user typed "Custp" — no match.
+        // `show data Custp` is a complete command naming a table
+        // that does not exist — surfaced by the ADR-0027
+        // diagnostic branch (the schema-existence ERROR).
         let cache = SchemaCache {
             tables: vec!["Customers".to_string()],
             ..SchemaCache::default()
@@ -966,8 +1003,8 @@ mod tests {
         match ambient_hint("show data Custp", 15, None, &cache) {
             Some(AmbientHint::Prose(p)) => {
                 assert!(
-                    p.contains("No such table"),
-                    "expected 'No such table' wording, got {p:?}",
+                    p.to_lowercase().contains("no such table"),
+                    "expected 'no such table' wording, got {p:?}",
                 );
                 assert!(p.contains("Custp"), "should name the bad ident, got {p:?}");
             }
@@ -975,6 +1012,84 @@ mod tests {
         }
     }
 
+    // ---- diagnostic hints (ADR-0027 hint wiring) ----
+
+    #[test]
+    fn ambient_hint_surfaces_unknown_table_diagnostic() {
+        use crate::dsl::types::Type;
+        let cache = schema_with_columns("Customers", &[("id", Type::Int)]);
+        match ambient_hint("show data Missing", 17, None, &cache) {
+            Some(AmbientHint::Prose(p)) => {
+                assert!(p.contains("Missing"), "got {p:?}");
+                assert!(
+                    p.to_lowercase().contains("no such table"),
+                    "got {p:?}",
+                );
+            }
+            other => panic!("expected Prose, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn ambient_hint_surfaces_type_mismatch_over_submit_prose() {
+        use crate::dsl::types::Type;
+        // The command parses cleanly — without the diagnostic
+        // branch this shows the misleading "press Enter" prose.
+        let cache = schema_with_columns("Events", &[("Count", Type::Int)]);
+        let input = "delete from Events where Count = 'oops'";
+        match ambient_hint(input, input.len(), None, &cache) {
+            Some(AmbientHint::Prose(p)) => {
+                assert!(!p.contains("Enter"), "should not invite submit: {p:?}");
+                assert!(p.contains("Count"), "should name the column: {p:?}");
+            }
+            other => panic!("expected Prose, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn ambient_hint_surfaces_like_numeric_warning() {
+        use crate::dsl::types::Type;
+        let cache = schema_with_columns("Events", &[("Count", Type::Int)]);
+        let input = "delete from Events where Count like '9%'";
+        match ambient_hint(input, input.len(), None, &cache) {
+            Some(AmbientHint::Prose(p)) => {
+                assert!(p.contains("LIKE"), "should mention LIKE: {p:?}");
+            }
+            other => panic!("expected Prose, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn ambient_hint_clean_command_still_invites_submit() {
+        use crate::dsl::types::Type;
+        let cache = schema_with_columns("Events", &[("Count", Type::Int)]);
+        let input = "delete from Events where Count = 7";
+        match ambient_hint(input, input.len(), None, &cache) {
+            Some(AmbientHint::Prose(p)) => {
+                assert!(p.contains("Enter"), "clean command invites submit: {p:?}");
+            }
+            other => panic!("expected Prose, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn ambient_hint_diagnostic_follows_the_cursor() {
+        use crate::dsl::types::Type;
+        // Two type-mismatch WARNINGs; the hint names the column
+        // whose offending literal the cursor sits in.
+        let cache =
+            schema_with_columns("Events", &[("a", Type::Int), ("b", Type::Int)]);
+        let input = "delete from Events where a = 'x' or b = 'y'";
+        let on_x = input.find("'x'").expect("'x' literal") + 1;
+        let on_y = input.find("'y'").expect("'y' literal") + 1;
+        let prose_at = |cursor| match ambient_hint(input, cursor, None, &cache) {
+            Some(AmbientHint::Prose(p)) => p,
+            other => panic!("expected Prose, got {other:?}"),
+        };
+        assert!(prose_at(on_x).contains("`a`"), "cursor on 'x' → column a");
+        assert!(prose_at(on_y).contains("`b`"), "cursor on 'y' → column b");
+    }
+
     #[test]
     fn ambient_hint_with_memo_carries_selected_index() {
         use crate::completion::{Candidate, CandidateKind, LastCompletion};