chore: remove dead code and stale doc claims found in multi-agent sweep

Compiler-verified via RUSTFLAGS=--force-warn dead_code plus workspace-wide
reference greps; three parallel audit agents covered the engine crate, the
other eight crates, and Copilot commit-message-vs-diff drift.

Removed:
- replay_overlay/input.rs: 19 orphaned twins (~950 lines) of items also
  defined in mod.rs — the glob re-export made the mod.rs copies win and
  the file-level #![allow(dead_code)] hid the corpses. The live keyboard/
  button handlers and ReplayScrubKeyHold stay; the allow is retired.
- retarget_animation (never called; doc examples were its only refs)
- ScanThemesRequestEvent (never registered/written/read; its doc claimed
  a handle_scan_themes consumer that does not exist)
- _VEC3_REFERENCED workaround const + now-unneeded Vec3 import
- solitaire_data: load_stats/save_stats/time_attack_session_with_now
  default-path wrappers (the _from/_to variants are the live API) and
  surplus re-export names (settings MIN/MAX bounds, token loaders)
- solitaire_core: Session re-export (no external consumer)
- solitaire_wasm: ReplayPlayer::is_finished (no JS caller)
- solitaire_app: build_app wrapper (real entry is run())

Doc fixes:
- audio_plugin: WAV count 5→7, add FoundationCompletedEvent table row,
  drop bogus 'placeholder' label, bevy_kira_audio→kira
- ToastVariant::Warning: variant is live (5 writer plugins); dropped the
  stale allow(dead_code) and its 'currently unused' comment

Deliberately kept: Spider module (staged forward work), WinCascadePlugin
(documented alternative cascade, pending owner decision), SyncCompleteEvent
and solitaire_sync ApiError/merge_at (§8 change-controlled, flagged to owner).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/events.rs

@@ -300,15 +300,6 @@ pub struct ForfeitEvent;
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct ForfeitRequestEvent;
 
-/// Fired when the player clicks "Scan for new themes" in Settings.
-///
-/// Consumed by `handle_scan_themes` in `SettingsPlugin`, which scans
-/// `user_theme_dir()` for `.zip` files, calls `import_theme()` on each
-/// unrecognised archive, refreshes [`crate::theme::ThemeRegistry`], and
-/// fires [`InfoToastEvent`] messages to report results.
-#[derive(Message, Debug, Clone, Copy, Default)]
-pub struct ScanThemesRequestEvent;
-
 /// Fired when the player requests a hint (H key). Carries the source card ID
 /// and destination pile for visual highlighting.
 ///