050b36391e
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