completion+hint: F1/F2 advanced-mode completion fixes

F1: the hint panel is the completion UI, so a premature "no such table/
column" ERROR on the token the user is still typing must not shadow its
completion. ambient_hint now suppresses an under-cursor error diagnostic
when a completion exists for the (non-empty) partial it overlaps, and
falls through to the candidates. Genuinely-unknown names (no prefix match)
still show the error; WARNINGs are unaffected. Both modes.

F2: projection-before-FROM ("select <cursor> from T" after deleting *)
offered the global column list instead of T's columns, because the §10.6
look-ahead's full-input walk can't reach FROM through an empty projection.
When the look-ahead finds no scope, retry with a neutral placeholder
inserted at the cursor so the trailing FROM/CTE scope is recovered for
narrowing. Only the repaired walk's from_scope/cte_bindings are used.

Test-first: 3 F1 tests (mid-typed completes, unknown still errors, simple-
mode DSL) + 1 F2 multi-table narrowing test. 1469 baseline green.

src/completion.rs

@@ -279,7 +279,33 @@ pub fn candidates_at_cursor_with_in_mode(
         && probe.cte_bindings.is_empty()
         && input.len() > leading.len()
     {
-        Some(crate::dsl::walker::completion_probe_in_mode(input, cache, mode))
+        let direct = crate::dsl::walker::completion_probe_in_mode(input, cache, mode);
+        if direct.from_scope.is_empty() && direct.cte_bindings.is_empty() {
+            // The slot at the cursor is empty/incomplete — e.g. the
+            // projection list of `select <cursor> from T` after the
+            // user deleted `*` — so the full-input walk never
+            // reached FROM and recovered no scope. Repair by
+            // inserting a neutral expression placeholder at the
+            // cursor and re-walking, so the trailing FROM/CTE scope
+            // is recovered for column narrowing (ADR-0032 §10.6).
+            // Only the repaired walk's `from_scope` / `cte_bindings`
+            // are consumed (table + columns), so the inserted token
+            // doesn't perturb the expected set, which comes from the
+            // leading probe.
+            let mut repaired = String::with_capacity(input.len() + 2);
+            repaired.push_str(&input[..start]);
+            repaired.push_str("1 ");
+            repaired.push_str(&input[start..]);
+            let repaired_probe =
+                crate::dsl::walker::completion_probe_in_mode(&repaired, cache, mode);
+            if repaired_probe.from_scope.is_empty() && repaired_probe.cte_bindings.is_empty() {
+                Some(direct)
+            } else {
+                Some(repaired_probe)
+            }
+        } else {
+            Some(direct)
+        }
     } else {
         None
     };