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/sync_plugin.rs

@@ -198,13 +198,17 @@ fn poll_pull_result(
             progress.0 = merged.progress;
             status.0 = SyncStatus::LastSynced(Utc::now());
         }
+        Err(SyncError::UnsupportedPlatform) => {
+            // No backend configured — not an error, just leave status as Idle.
+            status.0 = SyncStatus::Idle;
+        }
         Err(e) => {
             warn!("sync pull failed: {e}");
             let msg = match &e {
                 SyncError::Network(_) => "Can't reach server — check your connection".to_string(),
                 SyncError::Auth(_) => "Login expired — tap Sync Now after re-logging in".to_string(),
                 SyncError::Serialization(_) => format!("Unexpected server response: {e}"),
-                SyncError::UnsupportedPlatform => "Sync not configured".to_string(),
+                SyncError::UnsupportedPlatform => unreachable!("handled above"),
             };
             status.0 = SyncStatus::Error(msg);
         }
@@ -233,13 +237,14 @@ fn push_on_exit(
 
     // Prefer an existing tokio runtime; fall back to futures_lite block_on
     // for environments (e.g. tests) that don't have one.
-    match tokio::runtime::Handle::try_current() {
-        Ok(handle) => {
-            let _ = handle.block_on(provider.push(&payload));
-        }
-        Err(_) => {
-            let _ = future::block_on(provider.push(&payload));
-        }
+    let result = match tokio::runtime::Handle::try_current() {
+        Ok(handle) => handle.block_on(provider.push(&payload)),
+        Err(_) => future::block_on(provider.push(&payload)),
+    };
+    if let Err(e) = result {
+        // Log push failures on exit so they appear in crash/log reports.
+        // We cannot surface them to the UI at this point (game loop is done).
+        warn!("sync push on exit failed: {e}");
     }
 }