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/ui_theme.rs

@@ -379,6 +379,17 @@ pub const MOTION_BUTTON_BLEND_SECS: f32 = 0.10;
 /// readout 1.0 → 1.1 → 1.0. 250 ms.
 pub const MOTION_SCORE_PULSE_SECS: f32 = 0.25;
 
+/// Foundation-completion flourish — when a King lands on a foundation
+/// pile (Ace → King, 13 cards), briefly scale the King card 1.0 →
+/// [`FOUNDATION_FLOURISH_PEAK_SCALE`] → 1.0 and tint the matching
+/// `PileMarker` gold. 400 ms.
+pub const MOTION_FOUNDATION_FLOURISH_SECS: f32 = 0.4;
+
+/// Peak scale magnification reached at the midpoint of the
+/// foundation-completion flourish. The triangular curve climbs from
+/// 1.0 at `t=0` to this value at `t=0.5` and back to 1.0 at `t=1.0`.
+pub const FOUNDATION_FLOURISH_PEAK_SCALE: f32 = 1.15;
+
 /// Loading-ellipsis cycle — `.`/`..`/`...` toggles every step.
 /// 400 ms.
 pub const MOTION_LOADING_TICK_SECS: f32 = 0.40;