feat(data,engine): persistent Settings + SFX volume hotkeys

- solitaire_data::Settings { sfx_volume, first_run_complete } with
  atomic JSON persistence and clamping sanitizer.
- SettingsPlugin (engine): [ / ] adjust SFX volume by 0.1, clamped;
  persists on change; emits SettingsChangedEvent. No-op at rails.
- AudioPlugin applies sfx_volume to kira's main track at startup
  and on every change so live tweaks take effect without restart.
- Brief "SFX: N%" toast on each change. Help cheat sheet updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/help_plugin.rs

@@ -54,6 +54,7 @@ fn spawn_help_screen(commands: &mut Commands) {
         "  S            Toggle stats / progression".to_string(),
         "  H or ?       Toggle this help".to_string(),
         "  Esc          Pause / resume".to_string(),
+        "  [ / ]        SFX volume down / up".to_string(),
         String::new(),
         "Press H or ? to close".to_string(),
     ];