walker: keep optional trailing flags completable after --

Typing `--` to start an optional trailing flag (`--create-fk`
on `add 1:n relationship`, `--cascade` on `drop column`,
`--force-conversion` / `--dont-convert` on `change column`)
made completion go empty: the trailing `--` turns the parse
into a trailing-junk Mismatch, and the Mismatch arm of the
completion expected-set resolution returned only `[EndOfInput]`
— the skipped optional-flag expectations, carried in
`tail_expected`, were dropped.

completion_probe and expected_at_input now merge `tail_expected`
into a Mismatch's expected set. `tail_expected` is empty for a
genuine mid-command mismatch, so this only adds the outer
shape's skipped trailing optionals — exactly the continuations
the trailing `--` is starting to type. This also resolves the
"wrong usage hint" symptom: with `--create-fk` offered as a
candidate, the hint panel shows candidates instead of falling
through to the parse-error usage block.

Audit outcome (the requested scan): usage_key_for_input was
verified correct for every multi-form command — add / drop /
show, including the digit-led `add 1:n relationship` form —
and is now regression-locked. The flag-completion fix covers
the whole optional-trailing-flag class.

6 tests (3 flag-completion, 3 usage-key). 1131 passing.

src/completion.rs

@@ -909,6 +909,48 @@ mod tests {
         assert!(cs.contains(&"--all-rows".to_string()), "got {cs:?}");
     }
 
+    #[test]
+    fn typed_dashes_still_offer_an_optional_trailing_flag() {
+        // Regression: `add 1:n relationship … [--create-fk]` —
+        // at a trailing space the flag is offered, but once the
+        // user typed `--` the trailing-junk Mismatch dropped the
+        // skipped optional's expectation and completion went
+        // empty. Both positions must offer `--create-fk`.
+        let at_space = cands("add 1:n relationship from X.a to Y.b ", 37);
+        assert!(
+            at_space.contains(&"--create-fk".to_string()),
+            "trailing space should offer --create-fk, got {at_space:?}",
+        );
+        let at_dashes = cands("add 1:n relationship from X.a to Y.b --", 39);
+        assert!(
+            at_dashes.contains(&"--create-fk".to_string()),
+            "typed `--` should still offer --create-fk, got {at_dashes:?}",
+        );
+    }
+
+    #[test]
+    fn typed_dashes_offer_the_optional_cascade_flag_on_drop_column() {
+        // The same optional-flag class: `drop column … [--cascade]`.
+        let at_dashes = cands("drop column from table T: c --", 30);
+        assert!(
+            at_dashes.contains(&"--cascade".to_string()),
+            "typed `--` should offer --cascade, got {at_dashes:?}",
+        );
+    }
+
+    #[test]
+    fn typed_dashes_offer_the_change_column_conversion_flags() {
+        // `change column … [--force-conversion | --dont-convert]`
+        // — the flags sit in a `Repeated { min: 0 }`; the same
+        // trailing-junk-Mismatch fix must surface them.
+        let at_dashes = cands("change column T: c (int) --", 27);
+        assert!(
+            at_dashes.contains(&"--force-conversion".to_string())
+                && at_dashes.contains(&"--dont-convert".to_string()),
+            "typed `--` should offer both conversion flags, got {at_dashes:?}",
+        );
+    }
+
     // ---- App-lifecycle command completion (round-5 fold-in) ----
 
     #[test]