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.

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__where_expression__complex_and_or_expression_parses@complex_and_or.snap

@@ -1,11 +1,11 @@
 ---
 source: tests/typing_surface/where_expression.rs
-description: "input=\"delete from Things where k > 1 and t like 'a%' or k = 9\" cursor=55"
+description: "input=\"delete from Things where k > 1 and note like 'a%' or k = 9\" cursor=58"
 expression: "& a"
 ---
 Assessment {
-    input: "delete from Things where k > 1 and t like 'a%' or k = 9",
-    cursor: 55,
+    input: "delete from Things where k > 1 and note like 'a%' or k = 9",
+    cursor: 58,
     state: Valid,
     hint: Some(
         Prose(