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

tests/typing_surface/mod.rs

@@ -250,6 +250,7 @@ fn command_kind_label(cmd: &rdbms_playground::dsl::Command) -> String {
         App(app) => match app {
             AppCommand::Quit => "App(Quit)".into(),
             AppCommand::Help { .. } => "App(Help)".into(),
+            AppCommand::Hint => "App(Hint)".into(),
             AppCommand::Rebuild => "App(Rebuild)".into(),
             AppCommand::Save => "App(Save)".into(),
             AppCommand::SaveAs => "App(SaveAs)".into(),