feat(engine): F3-toggleable FPS / frame-time overlay

Performance work for the upcoming Android port needs a numeric
baseline we can quote across desktop and mobile, instead of "feels
slow". DiagnosticsHudPlugin wraps Bevy's FrameTimeDiagnosticsPlugin
and renders a tiny corner readout the developer can toggle with F3.

- Hidden by default — production builds ship the plugin but the
  overlay starts invisible.
- F3 reads ButtonInput<KeyCode> directly (not gated by pause /
  modal state); diagnostics should always be reachable.
- Reads `smoothed()` FPS + frame_time so the cell isn't a jittery
  per-frame scoreboard. Format: "FPS NN \u{2022} M.MM ms".
- Anchored top-right at z = Z_SPLASH + 100 so the readout sits
  above every modal / toast / splash layer.
- Update system bails when hidden so we don't pay the
  diagnostic-store lookup or text mutation when nobody's looking.

Next up on the perf track: get the Android build target wired so we
can put real numbers in this readout from a phone or emulator.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_app/src/main.rs

@@ -10,10 +10,10 @@ use solitaire_data::{load_settings_from, provider_for_backend, settings_file_pat
 use solitaire_engine::{
     register_theme_asset_sources, AchievementPlugin, AnimationPlugin, AssetSourcesPlugin,
     AudioPlugin, AutoCompletePlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
-    CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin, FontPlugin, GamePlugin, HelpPlugin,
-    HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin, OnboardingPlugin, PausePlugin,
-    ProfilePlugin, ProgressPlugin, RadialMenuPlugin, ReplayOverlayPlugin, ReplayPlaybackPlugin,
-    SelectionPlugin, SettingsPlugin, SplashPlugin,
+    CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, FeedbackAnimPlugin, FontPlugin,
+    GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
+    OnboardingPlugin, PausePlugin, ProfilePlugin, ProgressPlugin, RadialMenuPlugin,
+    ReplayOverlayPlugin, ReplayPlaybackPlugin, SelectionPlugin, SettingsPlugin, SplashPlugin,
     StatsPlugin, SyncPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin, TimeAttackPlugin,
     UiFocusPlugin, UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
 };
@@ -145,7 +145,8 @@ fn main() {
         .add_plugins(UiModalPlugin)
         .add_plugins(UiFocusPlugin)
         .add_plugins(UiTooltipPlugin)
-        .add_plugins(SplashPlugin);
+        .add_plugins(SplashPlugin)
+        .add_plugins(DiagnosticsHudPlugin);
 
     // Smart default window sizing: when no saved geometry was loaded,
     // resize the freshly-opened 1280×800 window to ~70 % of the primary