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/ddl.rs

@@ -968,6 +968,7 @@ pub static DROP: CommandNode = CommandNode {
     shape: DROP_SHAPE,
     ast_builder: build_drop,
     help_id: Some("ddl.drop"),
+    hint_id: None,
     usage_ids: &[
         "parse.usage.drop_table",
         "parse.usage.drop_column",
@@ -981,6 +982,7 @@ pub static ADD: CommandNode = CommandNode {
     shape: ADD_SHAPE,
     ast_builder: build_add,
     help_id: Some("ddl.add"),
+    hint_id: None,
     usage_ids: &[
         "parse.usage.add_column",
         "parse.usage.add_relationship",
@@ -993,6 +995,7 @@ pub static RENAME: CommandNode = CommandNode {
     shape: RENAME_COLUMN,
     ast_builder: build_rename_column,
     help_id: Some("ddl.rename"),
+    hint_id: None,
     usage_ids: &["parse.usage.rename_column"],};
 
 pub static CHANGE: CommandNode = CommandNode {
@@ -1000,6 +1003,7 @@ pub static CHANGE: CommandNode = CommandNode {
     shape: CHANGE_COLUMN,
     ast_builder: build_change_column,
     help_id: Some("ddl.change"),
+    hint_id: None,
     usage_ids: &["parse.usage.change_column"],};
 
 // =================================================================
@@ -1360,6 +1364,7 @@ pub static CREATE: CommandNode = CommandNode {
     shape: CREATE_TABLE,
     ast_builder: build_create_table,
     help_id: Some("ddl.create"),
+    hint_id: None,
     usage_ids: &["parse.usage.create_table"],};
 
 // =================================================================
@@ -1428,6 +1433,7 @@ pub static CREATE_M2N: CommandNode = CommandNode {
     shape: CREATE_M2N_SHAPE,
     ast_builder: build_create_m2n,
     help_id: Some("ddl.create_m2n"),
+    hint_id: None,
     usage_ids: &["parse.usage.create_m2n"],
 };
 
@@ -1858,6 +1864,7 @@ pub static SQL_CREATE_TABLE: CommandNode = CommandNode {
     shape: Node::Subgrammar(&super::sql_create_table::SQL_CREATE_TABLE_SHAPE),
     ast_builder: build_sql_create_table,
     help_id: Some("ddl.sql_create_table"),
+    hint_id: None,
     usage_ids: &["parse.usage.sql_create_table"],
 };
 
@@ -1877,6 +1884,7 @@ pub static SQL_DROP_TABLE: CommandNode = CommandNode {
     shape: SQL_DROP_TABLE_SHAPE,
     ast_builder: build_sql_drop_table,
     help_id: Some("ddl.sql_drop_table"),
+    hint_id: None,
     usage_ids: &["parse.usage.sql_drop_table"],
 };
 
@@ -1896,6 +1904,7 @@ pub static SQL_DROP_INDEX: CommandNode = CommandNode {
     shape: SQL_DROP_INDEX_SHAPE,
     ast_builder: build_sql_drop_index,
     help_id: Some("ddl.sql_drop_index"),
+    hint_id: None,
     usage_ids: &["parse.usage.sql_drop_index"],
 };
 
@@ -1977,6 +1986,7 @@ pub static SQL_CREATE_INDEX: CommandNode = CommandNode {
     shape: SQL_CREATE_INDEX_SHAPE,
     ast_builder: build_sql_create_index,
     help_id: Some("ddl.sql_create_index"),
+    hint_id: None,
     usage_ids: &["parse.usage.sql_create_index"],
 };
 
@@ -2535,6 +2545,7 @@ pub static SQL_ALTER_TABLE: CommandNode = CommandNode {
     shape: SQL_ALTER_TABLE_SHAPE,
     ast_builder: build_sql_alter_table,
     help_id: Some("ddl.sql_alter_table"),
+    hint_id: None,
     usage_ids: &["parse.usage.sql_alter_table"],
 };