Walker: node-attached HintMode via Node::Hinted (ADR-0024 §HintMode-per-node)

Replaces the hint resolver's signature-matching (does the expected set
contain all five literal forms? an Ident{NewName}?) with a grammar-
declared annotation. New Node::Hinted { mode, inner } wrapper; the
walker records the mode in WalkContext::pending_hint_mode on entry and
clears it on any successful match (cursor moved past the slot — this
also undoes the leak where a failed Hinted branch of a Choice would
otherwise strand a stale mode). The resolver reads pending_hint_mode
directly.

Value-literal fallback slots carry ProseOnly; NewName ident slots carry
ForceProse. hint_mode_at_input_inner now delegates to
hint_resolution_at_input — one resolution path, no duplicated logic.
No behaviour change; the typing-surface matrix guards it.

src/dsl/walker/context.rs

@@ -54,6 +54,13 @@ pub struct WalkContext<'a> {
     /// Cleared on successful inner match alongside
     /// `pending_value_type`.
     pub pending_value_column: Option<String>,
+    /// The hint-panel `HintMode` declared by the grammar node
+    /// the walker is currently inside (ADR-0024
+    /// §HintMode-per-node). Set on entry to a `Node::Hinted`
+    /// wrapper, cleared on successful inner match. The hint
+    /// resolver reads this directly instead of inferring the
+    /// slot kind from the shape of the expected set.
+    pub pending_hint_mode: Option<crate::dsl::grammar::HintMode>,
     /// The columns the user explicitly listed in
     /// `insert into <T> (col1, col2, …) values (…)` (Form A),
     /// in declaration order.
@@ -91,6 +98,7 @@ impl<'a> WalkContext<'a> {
             current_column: None,
             pending_value_type: None,
             pending_value_column: None,
+            pending_hint_mode: None,
             user_listed_columns: None,
         }
     }