feat/fix/perf(engine,data,assetgen): ambient audio, sync bug fixes, hot-path cleanup

**ambient_loop.wav (task 5)**
- solitaire_assetgen: add ambient_loop() synthesizer — 5 s seamless loop,
  55 Hz drone with 2nd/3rd harmonics, 0.2 Hz LFO breath, 16-bit mono 44100 Hz
- audio_plugin: load ambient_loop.wav via include_bytes!() replacing the
  card_flip.wav placeholder; decouple start_ambient_loop() from SoundLibrary

**sync bug fixes (task 11)**
- sync_plugin: LocalOnlyProvider returning UnsupportedPlatform now sets
  SyncStatus::Idle instead of displaying a misleading "Sync not configured" error
- sync_client: extract_pull_body / extract_push_body now return SyncError::Auth
  only for HTTP 401/403; all other non-2xx statuses return SyncError::Network
- sync_plugin: push_on_exit now logs a warn! on failure instead of silently
  discarding the result

**hot-path performance (task 12)**
- card_plugin: card_positions() now returns &Card references (lifetime-bound to
  GameState) instead of owned Card clones — eliminates 52 Card clones per
  sync_cards() call (runs every animation frame)
- input_plugin: card_position() takes &PileType instead of PileType, eliminating
  PileType copies at every drag hit-test call site
- animation_plugin: eliminate intermediate AnimSpeed clone in handle_win_cascade()

**docs (tasks 11, 13)**
- docs/sync_test_runbook.md: manual test runbook for cross-machine sync
- docs/android_investigation.md: cargo-mobile2 port investigation and effort estimate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/card_plugin.rs

@@ -265,18 +265,18 @@ fn sync_cards(
         match existing.get(&card.id) {
             Some(&(entity, cur)) => {
                 update_card_entity(
-                    &mut commands, entity, &card, position, z, layout,
+                    &mut commands, entity, card, position, z, layout,
                     slide_secs, back_colour, color_blind, cur,
                 )
             }
-            None => spawn_card_entity(&mut commands, &card, position, z, layout, back_colour, color_blind),
+            None => spawn_card_entity(&mut commands, card, position, z, layout, back_colour, color_blind),
         }
     }
 }
 
 /// Returns an ordered vec of (card, position, z) for every card in the game.
-fn card_positions(game: &GameState, layout: &Layout) -> Vec<(Card, Vec2, f32)> {
-    let mut out: Vec<(Card, Vec2, f32)> = Vec::with_capacity(52);
+fn card_positions<'a>(game: &'a GameState, layout: &Layout) -> Vec<(&'a Card, Vec2, f32)> {
+    let mut out: Vec<(&'a Card, Vec2, f32)> = Vec::with_capacity(52);
     let piles = [
         PileType::Stock,
         PileType::Waste,
@@ -331,7 +331,7 @@ fn card_positions(game: &GameState, layout: &Layout) -> Vec<(Card, Vec2, f32)> {
             };
             let pos = Vec2::new(base.x + x_offset, base.y + y_offset);
             let z = 1.0 + (slot as f32) * STACK_FAN_FRAC;
-            out.push((card.clone(), pos, z));
+            out.push((card, pos, z));
             if is_tableau {
                 let step = if card.face_up {
                     TABLEAU_FAN_FRAC