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/app.rs

@@ -9,7 +9,8 @@
 
 use crate::dsl::command::{AppCommand, Command, MessagesValue, ModeValue};
 use crate::dsl::grammar::{
-    CommandNode, IdentSource, IdentValidator, Node, ValidationError, Word,
+    CommandNode, HintMode, IdentSource, IdentValidator, Node, ValidationError,
+    Word,
 };
 use crate::dsl::walker::outcome::{MatchedKind, MatchedPath};
 
@@ -43,17 +44,23 @@ const UNKNOWN_MESSAGES_VALIDATOR: IdentValidator = validate_unknown_messages;
 
 const SAVE_AS_WORD: Node = Node::Word(Word::keyword("as"));
 
+const IMPORT_TARGET_IDENT: Node = Node::Ident {
+    source: IdentSource::NewName,
+    role: "target",
+    validator: None,
+    highlight_override: None,
+    writes_table: false,
+    writes_column: false,
+    writes_user_listed_column: false,
+};
+const IMPORT_TARGET: Node = Node::Hinted {
+    mode: HintMode::ForceProse("hint.ambient_typing_name"),
+    inner: &IMPORT_TARGET_IDENT,
+};
+
 const IMPORT_AS_TARGET: Node = Node::Seq(&[
     Node::Word(Word::keyword("as")),
-    Node::Ident {
-        source: IdentSource::NewName,
-        role: "target",
-        validator: None,
-        highlight_override: None,
-        writes_table: false,
-        writes_column: false,
-        writes_user_listed_column: false,
-    },
+    IMPORT_TARGET,
 ]);
 const IMPORT_AS_TARGET_OPT: Node = Node::Optional(&IMPORT_AS_TARGET);