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/audio_plugin.rs

@@ -1,7 +1,7 @@
 //! Sound-effect playback via `kira`.
 //!
-//! Loads five embedded WAVs (`include_bytes!`) at startup and plays them in
-//! response to gameplay events:
+//! Loads seven embedded WAVs (`include_bytes!`) at startup — six SFX plus
+//! the ambient loop — and plays them in response to gameplay events:
 //!
 //! | Event | Sound |
 //! |---|---|
@@ -10,6 +10,7 @@
 //! | `MoveRejectedEvent` | `card_invalid.wav` |
 //! | `NewGameRequestEvent` | `card_deal.wav` |
 //! | `GameWonEvent` | `win_fanfare.wav` |
+//! | `FoundationCompletedEvent` | `foundation_complete.wav` |
 //!
 //! An ambient loop (`ambient_loop.wav`) is started at plugin startup at very
 //! low volume (0.05 amplitude) routed through `music_track`.
@@ -38,7 +39,7 @@ use crate::settings_plugin::{SettingsChangedEvent, SettingsResource};
 /// Volume amplitude for the stock-recycle draw sound (half of normal 1.0).
 const RECYCLE_VOLUME: f64 = 0.5;
 
-/// Volume amplitude for the ambient music loop placeholder.
+/// Volume amplitude for the ambient music loop.
 const AMBIENT_VOLUME: f64 = 0.05;
 
 /// Converts a linear amplitude (0.0–1.0+) to the `Decibels` type used by
@@ -101,7 +102,7 @@ pub struct MuteState {
     pub music_muted: bool,
 }
 
-/// Plays sound effects and background music via `bevy_kira_audio`. Responds to game events (card place, flip, invalid move, win fanfare) and respects volume settings from `SettingsResource`.
+/// Plays sound effects and background music via `kira`. Responds to game events (card place, flip, invalid move, win fanfare) and respects volume settings from `SettingsResource`.
 pub struct AudioPlugin;
 
 impl Plugin for AudioPlugin {