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/grammar/mod.rs

@@ -308,6 +308,24 @@ pub enum Node {
         column_name: Option<&'static str>,
         inner: &'static Self,
     },
+    /// Annotates `inner` with a hint-panel `HintMode` (ADR-0024
+    /// §HintMode-per-node). On entry the walker records `mode`
+    /// in `WalkContext::pending_hint_mode`; on a successful
+    /// inner match the record clears (so positions past the
+    /// slot don't carry stale hint state). Transparent to
+    /// matching, highlighting and the expected-set otherwise —
+    /// it walks `inner` and returns its result verbatim.
+    ///
+    /// This is the node-attached replacement for the hint
+    /// resolver's earlier signature-matching: the grammar tree
+    /// declares the hint mode at the slot, the walker
+    /// propagates it, the resolver reads it. Used by the
+    /// value-literal fallback slot (`ProseOnly`) and `NewName`
+    /// ident slots (`ForceProse`).
+    Hinted {
+        mode: HintMode,
+        inner: &'static Self,
+    },
 }
 
 /// Top-level entry record. One per command. The `entry` keyword