feat(engine): foundation completion flourish — King-on-foundation celebration

Now that foundations are unlocked and "completing" one is a real
moment (rather than a foregone conclusion based on suit assignment),
each Ace-through-King run gets its own small celebration when the
King lands.

Three layers fire on a single FoundationCompletedEvent emitted by
game_plugin's handle_move when a successful move leaves a
PileType::Foundation pile holding 13 cards:

1. King card scale-pulse via a new FoundationFlourish component.
   Triangular curve 1.0 → 1.15 → 1.0 over MOTION_FOUNDATION_FLOURISH
   _SECS (0.4s) — same shape as the existing ScorePulse so the feel
   matches.
2. Pile-marker tint flourish via FoundationMarkerFlourish — the
   foundation marker's sprite colour lerps to STATE_SUCCESS for the
   first half of the duration then fades back. Reuses the existing
   success-signal palette; no new colour token.
3. Audio cue: foundation_complete.wav, a synthesised C6→E6→G6 triad
   with 2nd-harmonic warmth and AR decay (~240 ms). Sits an octave
   above win_fanfare's root so the layered fourth-completion + win
   cascade reads cleanly. Generated via solitaire_assetgen's
   foundation_complete() function and embedded via include_bytes!().

The visual systems run .after(GameMutation) so the post-move pile
state is visible when the King is identified. Both flourish
components remove themselves once elapsed time exceeds duration —
no animation queue or scheduler integration needed.

Pure foundation_flourish_scale(elapsed, duration) helper is
unit-tested for the curve, edge clamps, and zero-duration safety.
Three integration tests on the firing logic verify the event fires
exactly once when a King completes a foundation, doesn't fire for
non-foundation moves, and doesn't fire when the foundation is at 12
cards.

The fourth completion still co-occurs with the win cascade — the
two layer cleanly because the flourish's scale is on the King card
sprite while the cascade is a screen-shake + per-card rotation, and
the foundation_complete ping is a higher octave than the win
fanfare's root.

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

solitaire_engine/src/lib.rs

@@ -69,8 +69,9 @@ pub use card_animation::{
     FrameTimeDiagnostics, DIAG_WINDOW_SIZE,
 };
 pub use feedback_anim_plugin::{
-    deal_stagger_delay, deal_stagger_secs_for_speed, shake_offset, settle_scale,
-    FeedbackAnimPlugin, SettleAnim, ShakeAnim,
+    deal_stagger_delay, deal_stagger_secs_for_speed, foundation_flourish_scale, shake_offset,
+    settle_scale, FeedbackAnimPlugin, FoundationFlourish, FoundationMarkerFlourish, SettleAnim,
+    ShakeAnim,
 };
 pub use auto_complete_plugin::AutoCompletePlugin;
 pub use audio_plugin::{AudioPlugin, AudioState, SoundLibrary};
@@ -82,8 +83,8 @@ pub use font_plugin::{FontPlugin, FontResource};
 pub use cursor_plugin::CursorPlugin;
 pub use events::{
     AchievementUnlockedEvent, CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent,
-    ForfeitEvent, ForfeitRequestEvent, GameWonEvent, HelpRequestEvent, HintVisualEvent,
-    InfoToastEvent, ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent,
+    ForfeitEvent, ForfeitRequestEvent, FoundationCompletedEvent, GameWonEvent, HelpRequestEvent,
+    HintVisualEvent, InfoToastEvent, ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent,
     NewGameConfirmEvent, NewGameRequestEvent, PauseRequestEvent, StartChallengeRequestEvent,
     StartDailyChallengeRequestEvent, StartTimeAttackRequestEvent, StartZenRequestEvent,
     StateChangedEvent, SyncCompleteEvent, ToggleAchievementsRequestEvent,