ADR-0024 round-5 follow-up: surface tail-Optional expectations

Pre-Phase-D, `save ` parsed as a complete `save` command, so
the completion engine had nothing to mine: `as` never surfaced
as a Tab candidate. This is the round-5 gap the handoffs have
been tracking.

`WalkResult` gains a `tail_expected: Vec<Expectation>` field.
The walker's top-level `Matched` branch copies the outer
shape's skipped-Optional expectations into it. `expected_at_input`
returns `tail_expected` on `Match` so the completion engine
sees the optional-suffix continuations and offers them as Tab
candidates.

`hint_mode_at_input` deliberately does NOT consume
`tail_expected` — surfacing prose like "Type a name" at the
end of a valid command would be misleading. A new private
`expected_for_hint` returns empty on `Match` to preserve this.
The split distinguishes "valid + could continue" (completion
helps) from "invalid + must continue" (hint resolver helps).

Tests:
- `save ` Tab → `as` (new test, the original round-5 gap).
- `messages ` Tab → `short` and `verbose` (same shape).
- Existing `hint_mode_none_for_complete_command` stays green
  because hint resolver ignores tail_expected.
- 830 total passing, 0 failing, 1 ignored. Clippy clean.

src/completion.rs

@@ -814,14 +814,27 @@ mod tests {
         assert!(cs.contains(&"advanced".to_string()), "got {cs:?}");
     }
 
-    // Note: `save ` and `messages ` are deliberately NOT tested
-    // here. Both commands accept their bare form as a valid parse
-    // — `save` opens the save modal, `messages` shows the current
-    // verbosity — so the parser returns Ok at those positions
-    // and the completion engine has no expected-set to mine. The
-    // optional-suffix candidates (`as`, `short`, `verbose`) would
-    // need a separate probe mechanism (deferred — same shape as
-    // the post-complete-parse gap for `--create-fk` etc.).
+    // ---- Optional-suffix completion (round-5 gap, closed in Phase D) ----
+    //
+    // Pre-Phase-D: `save ` parsed as a valid `save` command, so
+    // the completion engine had no expected-set to mine and the
+    // `as` suffix never surfaced as a Tab candidate. Phase D's
+    // `WalkResult::tail_expected` carries the outer shape's
+    // skipped-Optional expectations even on `Match`, so these
+    // surface without a separate probe mechanism.
+
+    #[test]
+    fn save_space_offers_as_via_tail_expected() {
+        let cs = cands("save ", 5);
+        assert_eq!(cs, vec!["as".to_string()]);
+    }
+
+    #[test]
+    fn messages_space_offers_short_and_verbose_via_tail_expected() {
+        let cs = cands("messages ", 9);
+        assert!(cs.contains(&"short".to_string()), "got {cs:?}");
+        assert!(cs.contains(&"verbose".to_string()), "got {cs:?}");
+    }
 
     // ---- Value-literal slot suppression (round-6) -----------
 

src/dsl/walker/mod.rs

@@ -52,7 +52,14 @@ 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);
+    // Hint mode is only meaningful at *required* slot positions
+    // (Incomplete / Mismatch outcomes). For complete commands
+    // (Match), `tail_expected` may carry optional-suffix
+    // expectations — completion surfaces those as Tab
+    // candidates, but the hint resolver should stay silent so
+    // we don't push prose like "Type a name" at the end of a
+    // valid command.
+    let expected = expected_for_hint(source);
     if expected.is_empty() {
         return None;
     }
@@ -125,18 +132,56 @@ pub fn expected_at_input(source: &str) -> Vec<outcome::Expectation> {
     }
     let mut ctx = context::WalkContext::new();
     let (result, _cmd) = walk(source, outcome::WalkBound::EndOfInput, &mut ctx);
-    match result.map(|r| r.outcome) {
-        Some(outcome::WalkOutcome::Match { .. }) => Vec::new(),
-        Some(outcome::WalkOutcome::Incomplete { expected, .. }) => expected,
-        Some(outcome::WalkOutcome::Mismatch { expected, .. }) => expected,
-        Some(outcome::WalkOutcome::ValidationFailed { .. }) => Vec::new(),
+    let Some(result) = result else {
         // Walker didn't engage (unknown entry word): the
         // completion engine should still surface the available
         // entry words so the user can recover.
-        None => REGISTRY
+        return REGISTRY
             .iter()
             .map(|c| outcome::Expectation::Word(c.entry.primary))
-            .collect(),
+            .collect();
+    };
+    match result.outcome {
+        // On Match, surface the outer-shape's skipped-Optional
+        // expectations so the completion engine can offer
+        // optional-suffix candidates at the end of a valid
+        // command (`save` → `as`, etc.).
+        outcome::WalkOutcome::Match { .. } => result.tail_expected,
+        outcome::WalkOutcome::Incomplete { expected, .. }
+        | outcome::WalkOutcome::Mismatch { expected, .. } => expected,
+        outcome::WalkOutcome::ValidationFailed { .. } => Vec::new(),
+    }
+}
+
+/// Strict-required expected set at the end of `source`. Like
+/// `expected_at_input` but returns empty on `WalkOutcome::Match`
+/// — optional-suffix continuations are not surfaced. Used by
+/// the hint resolver to distinguish "must type more" from
+/// "could continue".
+#[must_use]
+fn expected_for_hint(source: &str) -> Vec<outcome::Expectation> {
+    use crate::dsl::grammar::REGISTRY;
+
+    if source.trim().is_empty() {
+        return REGISTRY
+            .iter()
+            .map(|c| outcome::Expectation::Word(c.entry.primary))
+            .collect();
+    }
+    let mut ctx = context::WalkContext::new();
+    let (result, _cmd) = walk(source, outcome::WalkBound::EndOfInput, &mut ctx);
+    let Some(result) = result else {
+        return REGISTRY
+            .iter()
+            .map(|c| outcome::Expectation::Word(c.entry.primary))
+            .collect();
+    };
+    match result.outcome {
+        outcome::WalkOutcome::Match { .. } | outcome::WalkOutcome::ValidationFailed { .. } => {
+            Vec::new()
+        }
+        outcome::WalkOutcome::Incomplete { expected, .. }
+        | outcome::WalkOutcome::Mismatch { expected, .. } => expected,
     }
 }
 
@@ -196,6 +241,7 @@ pub fn walk<'a>(
         class: grammar::HighlightClass::Keyword,
     });
 
+    let mut tail_expected: Vec<Expectation> = Vec::new();
     let outcome = match walk_node(
         effective_source,
         kw_end,
@@ -204,6 +250,21 @@ pub fn walk<'a>(
         &mut path,
         &mut per_byte,
     ) {
+        NodeWalkResult::Matched { end, skipped } => {
+            // Carry the outer shape's skipped-Optional
+            // expectations into WalkResult so completion can
+            // surface optional-suffix candidates (`save` →
+            // `as`). Empty for shapes with no trailing
+            // optionals.
+            tail_expected = skipped;
+            NodeWalkResult::Matched {
+                end,
+                skipped: Vec::new(),
+            }
+        }
+        other => other,
+    };
+    let outcome = match outcome {
         NodeWalkResult::Matched { end, .. } => {
             let trailing = skip_whitespace(effective_source, end);
             if trailing < effective_source.len() {
@@ -272,6 +333,7 @@ pub fn walk<'a>(
         outcome: final_outcome,
         matched_path: path,
         per_byte_class: per_byte,
+        tail_expected,
     };
     (Some(result), cmd)
 }

src/dsl/walker/outcome.rs

@@ -164,4 +164,16 @@ pub struct WalkResult {
     pub outcome: WalkOutcome,
     pub matched_path: MatchedPath,
     pub per_byte_class: Vec<ByteClass>,
+    /// Optional-Optional expectations the walker could have
+    /// accepted but didn't because the outer shape ran out at a
+    /// node boundary (ADR-0024 §architecture, round-5 follow-up).
+    ///
+    /// Populated on `WalkOutcome::Match` so completion can offer
+    /// optional-suffix candidates at the end of a valid command
+    /// — e.g., after typing `save` the walker matches the
+    /// Optional `as` as skipped, the suffix carries it here, and
+    /// the completion engine surfaces `as` as a Tab candidate.
+    /// Empty on the non-Match outcomes — those carry expected
+    /// information inside the outcome variant itself.
+    pub tail_expected: Vec<Expectation>,
 }