feat: ADR-0036 Phase 3b — live typed-slot hints + highlighting for INSERT VALUES

Give each positional INSERT VALUES position its column identity so a lone
literal gets the column-typed slot (live per-column hint + mismatch
highlight) and any expression falls through to sql_expr — completing the
typed-DML-values feature for the INSERT surface (single/multi-row, Form A
and Form B).

New zero-width Node::SetColumn(&TableColumn) primitive establishes the
active column for the value position that follows (sets current_column +
pending_value_column, like an Ident{writes_column} but without consuming
input); a DynamicSubgrammar emits SetColumn(col) + the shared SET_VALUE
per position. Column mapping mirrors do_sql_insert: Form A → listed
columns; Form B → all columns in declaration order (advanced-mode Form B
auto-fills nothing; an omitted shortid in Form A is auto-filled and has no
VALUES position).

Reconcile with the per-tuple arity diagnostic (ADR-0033 §8.1): a
fixed-length typed Seq would reject wrong-arity tuples and suppress that
post-walk diagnostic, so the tuple value list is an arity-gating lookahead
— a correct-arity tuple uses the typed Seq; a wrong-arity tuple keeps the
type-blind sql_expr repeat so §8.1 fires unchanged. Correct-arity tuples
get full live feedback, including a wrong-kind literal like 'text' into an
int column.

Records ADR-0036 Amendment 1 (Phase 3b detail + the arity reconciliation);
ADR-0036 is now fully implemented.

Tests: 1947 passing (+8), 0 failed, 0 skipped, 1 ignored; clippy clean.

src/dsl/walker/driver.rs

@@ -250,6 +250,19 @@ fn walk_node_inner(
                 Box::leak(Box::new(factory(ctx, source, pos)));
             walk_node(source, pos, resolved, ctx, path, per_byte)
         }
+        Node::SetColumn(col) => {
+            // ADR-0036 Phase 3b: zero-width — establish the active
+            // column for the value position that follows, exactly as an
+            // `Ident { writes_column: true }` would (current_column for
+            // the typed slot's dispatch; pending_value_column for the
+            // hint's "for `col`:" framing), but without consuming a
+            // column identifier (VALUES positions are positional). The
+            // following `SET_VALUE` slot reads `current_column`.
+            let col: &crate::completion::TableColumn = col;
+            ctx.current_column = Some(col.clone());
+            ctx.pending_value_column = Some(col.name.clone());
+            NodeWalkResult::Matched { end: pos, skipped: Vec::new() }
+        }
         Node::TypedValueSlot {
             ty,
             column_name,