feat(hint): H2 Phase A — hint command + F1 keybinding skeleton (ADR-0053)

The mechanism for the contextual hint, with tier-2 fallback; the
tier-3 corpus lands in later phases.

- new CommandNode `hint_id` field (all None for now)
- AppCommand::Hint + HINT grammar node + REGISTRY + dispatch
- F1 read-only overlay in handle_key (buffer/cursor/memo untouched)
- note_hint* renderers; hint_id_for_input_in_mode (shared selection
  helper refactored out of usage_keys_for_input_in_mode)
- last_error_hint_key + friendly::error_hint_class classifier
- catalogue: help.app.hint / parse.usage.hint / hint.getting_started
- +12 tests; 2483 pass / 1 ignored, clippy clean

src/dsl/grammar/app.rs

@@ -177,6 +177,9 @@ const fn build_rebuild(_path: &MatchedPath, _source: &str) -> Result<Command, Va
 const fn build_undo(_path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
     Ok(Command::App(AppCommand::Undo))
 }
+const fn build_hint(_path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
+    Ok(Command::App(AppCommand::Hint))
+}
 
 const fn build_redo(_path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
     Ok(Command::App(AppCommand::Redo))
@@ -263,6 +266,7 @@ pub static QUIT: CommandNode = CommandNode {
     shape: EMPTY_SEQ,
     ast_builder: build_quit,
     help_id: Some("app.quit"),
+    hint_id: None,
     usage_ids: &["parse.usage.quit"],};
 
 pub static HELP: CommandNode = CommandNode {
@@ -270,13 +274,24 @@ pub static HELP: CommandNode = CommandNode {
     shape: HELP_TOPIC_OPT,
     ast_builder: build_help,
     help_id: Some("app.help"),
+    hint_id: None,
     usage_ids: &["parse.usage.help"],};
 
+pub static HINT: CommandNode = CommandNode {
+    entry: Word::keyword("hint"),
+    shape: EMPTY_SEQ,
+    ast_builder: build_hint,
+    help_id: Some("app.hint"),
+    // hint_id assigned in Phase C with the tier-3 corpus (ADR-0053).
+    hint_id: None,
+    usage_ids: &["parse.usage.hint"],};
+
 pub static REBUILD: CommandNode = CommandNode {
     entry: Word::keyword("rebuild"),
     shape: EMPTY_SEQ,
     ast_builder: build_rebuild,
     help_id: Some("app.rebuild"),
+    hint_id: None,
     usage_ids: &["parse.usage.rebuild"],};
 
 pub static SAVE: CommandNode = CommandNode {
@@ -284,6 +299,7 @@ pub static SAVE: CommandNode = CommandNode {
     shape: SAVE_AS_OPT,
     ast_builder: build_save,
     help_id: Some("app.save"),
+    hint_id: None,
     usage_ids: &["parse.usage.save"],};
 
 pub static NEW: CommandNode = CommandNode {
@@ -291,6 +307,7 @@ pub static NEW: CommandNode = CommandNode {
     shape: EMPTY_SEQ,
     ast_builder: build_new,
     help_id: Some("app.new"),
+    hint_id: None,
     usage_ids: &["parse.usage.new"],};
 
 pub static LOAD: CommandNode = CommandNode {
@@ -298,6 +315,7 @@ pub static LOAD: CommandNode = CommandNode {
     shape: EMPTY_SEQ,
     ast_builder: build_load,
     help_id: Some("app.load"),
+    hint_id: None,
     usage_ids: &["parse.usage.load"],};
 
 pub static EXPORT: CommandNode = CommandNode {
@@ -305,6 +323,7 @@ pub static EXPORT: CommandNode = CommandNode {
     shape: EXPORT_PATH_OPT,
     ast_builder: build_export,
     help_id: Some("app.export"),
+    hint_id: None,
     usage_ids: &["parse.usage.export"],};
 
 pub static IMPORT: CommandNode = CommandNode {
@@ -312,6 +331,7 @@ pub static IMPORT: CommandNode = CommandNode {
     shape: IMPORT_BODY_OPT,
     ast_builder: build_import,
     help_id: Some("app.import"),
+    hint_id: None,
     usage_ids: &["parse.usage.import"],};
 
 pub static MODE: CommandNode = CommandNode {
@@ -319,6 +339,7 @@ pub static MODE: CommandNode = CommandNode {
     shape: MODE_VALUE,
     ast_builder: build_mode,
     help_id: Some("app.mode"),
+    hint_id: None,
     usage_ids: &["parse.usage.mode"],};
 
 pub static MESSAGES: CommandNode = CommandNode {
@@ -326,6 +347,7 @@ pub static MESSAGES: CommandNode = CommandNode {
     shape: MESSAGES_VALUE_OPT,
     ast_builder: build_messages,
     help_id: Some("app.messages"),
+    hint_id: None,
     usage_ids: &["parse.usage.messages"],};
 
 pub static UNDO: CommandNode = CommandNode {
@@ -333,6 +355,7 @@ pub static UNDO: CommandNode = CommandNode {
     shape: EMPTY_SEQ,
     ast_builder: build_undo,
     help_id: Some("app.undo"),
+    hint_id: None,
     usage_ids: &["parse.usage.undo"],};
 
 pub static REDO: CommandNode = CommandNode {
@@ -340,6 +363,7 @@ pub static REDO: CommandNode = CommandNode {
     shape: EMPTY_SEQ,
     ast_builder: build_redo,
     help_id: Some("app.redo"),
+    hint_id: None,
     usage_ids: &["parse.usage.redo"],};
 
 pub static COPY: CommandNode = CommandNode {
@@ -347,4 +371,5 @@ pub static COPY: CommandNode = CommandNode {
     shape: COPY_VALUE_OPT,
     ast_builder: build_copy,
     help_id: Some("app.copy"),
+    hint_id: None,
     usage_ids: &["parse.usage.copy"],};