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_engine/src/lib.rs

@@ -12,6 +12,7 @@ pub mod feedback_anim_plugin;
 pub mod challenge_plugin;
 pub mod cursor_plugin;
 pub mod daily_challenge_plugin;
+pub mod diagnostics_hud;
 pub mod events;
 pub mod game_plugin;
 pub mod help_plugin;
@@ -85,6 +86,7 @@ pub use card_plugin::{
 };
 pub use font_plugin::{FontPlugin, FontResource};
 pub use cursor_plugin::CursorPlugin;
+pub use diagnostics_hud::DiagnosticsHudPlugin;
 pub use events::{
     AchievementUnlockedEvent, CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent,
     ForfeitEvent, ForfeitRequestEvent, FoundationCompletedEvent, GameWonEvent, HelpRequestEvent,