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/audio_plugin.rs

@@ -28,8 +28,8 @@ use kira::track::{TrackBuilder, TrackHandle};
 use kira::{AudioManager, AudioManagerSettings, Decibels, DefaultBackend, Tween, Value};
 
 use crate::events::{
-    CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent, GameWonEvent, MoveRejectedEvent,
-    MoveRequestEvent, NewGameRequestEvent, UndoRequestEvent,
+    CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent, FoundationCompletedEvent,
+    GameWonEvent, MoveRejectedEvent, MoveRequestEvent, NewGameRequestEvent, UndoRequestEvent,
 };
 use crate::pause_plugin::PausedResource;
 use crate::resources::GameStateResource;
@@ -70,6 +70,12 @@ pub struct SoundLibrary {
     pub place: StaticSoundData,
     pub invalid: StaticSoundData,
     pub fanfare: StaticSoundData,
+    /// Per-suit foundation-completion ping. Played whenever a King
+    /// lands on a foundation pile (Ace → King, 13 cards). ~240 ms,
+    /// rising C-major triad an octave above `fanfare`'s root so the
+    /// two layer cleanly when the fourth completion co-occurs with
+    /// the win cascade.
+    pub foundation_complete: StaticSoundData,
 }
 
 /// Wraps the audio backend. `NonSend` because cpal streams are `!Send` on
@@ -145,6 +151,7 @@ impl Plugin for AudioPlugin {
             .add_message::<CardFlippedEvent>()
             .add_message::<CardFaceRevealedEvent>()
             .add_message::<UndoRequestEvent>()
+            .add_message::<FoundationCompletedEvent>()
             .add_message::<SettingsChangedEvent>()
             .add_systems(Startup, apply_initial_volume)
             .add_systems(
@@ -157,6 +164,7 @@ impl Plugin for AudioPlugin {
                     play_on_win,
                     play_on_face_revealed,
                     play_on_undo,
+                    play_on_foundation_complete,
                     apply_volume_on_change,
                     handle_mute_keys,
                 ),
@@ -170,12 +178,15 @@ fn build_library() -> Option<SoundLibrary> {
     let place = decode(include_bytes!("../../assets/audio/card_place.wav"))?;
     let invalid = decode(include_bytes!("../../assets/audio/card_invalid.wav"))?;
     let fanfare = decode(include_bytes!("../../assets/audio/win_fanfare.wav"))?;
+    let foundation_complete =
+        decode(include_bytes!("../../assets/audio/foundation_complete.wav"))?;
     Some(SoundLibrary {
         deal,
         flip,
         place,
         invalid,
         fanfare,
+        foundation_complete,
     })
 }
 
@@ -451,6 +462,25 @@ fn play_on_face_revealed(
     }
 }
 
+/// Plays the per-suit completion ping whenever a `FoundationCompletedEvent`
+/// fires (a King lands on a foundation pile that now holds Ace → King).
+///
+/// The fourth firing co-occurs with `GameWonEvent` and the win fanfare;
+/// the two layer cleanly because the ping sits an octave above the
+/// fanfare's root and is much shorter (~240 ms vs ~970 ms).
+fn play_on_foundation_complete(
+    mut events: MessageReader<FoundationCompletedEvent>,
+    mut audio: NonSendMut<AudioState>,
+    lib: Option<Res<SoundLibrary>>,
+) {
+    let Some(lib) = lib else {
+        return;
+    };
+    for _ in events.read() {
+        play(&mut audio, &lib.foundation_complete);
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;