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_data/src/storage.rs

@@ -46,22 +46,6 @@ pub fn save_stats_to(path: &Path, stats: &StatsSnapshot) -> io::Result<()> {
     Ok(())
 }
 
-/// Load stats from the platform default path. Returns default if the path
-/// is unavailable or the file is missing/corrupt.
-pub fn load_stats() -> StatsSnapshot {
-    stats_file_path()
-        .map(|p| load_stats_from(&p))
-        .unwrap_or_default()
-}
-
-/// Save stats to the platform default path. Returns an error if the platform
-/// data dir is unavailable or the write fails.
-pub fn save_stats(stats: &StatsSnapshot) -> io::Result<()> {
-    let path = stats_file_path()
-        .ok_or_else(|| io::Error::new(io::ErrorKind::NotFound, "platform data dir unavailable"))?;
-    save_stats_to(&path, stats)
-}
-
 // ---------------------------------------------------------------------------
 // In-progress game state
 // ---------------------------------------------------------------------------
@@ -245,18 +229,6 @@ pub fn delete_time_attack_session_at(path: &Path) -> io::Result<()> {
     }
 }
 
-/// Convenience helper for callers that want to stamp a session with the
-/// current wall-clock time. Equivalent to constructing the struct
-/// manually and setting `saved_at_unix_secs` to `SystemTime::now()`.
-pub fn time_attack_session_with_now(remaining_secs: f32, wins: u32) -> TimeAttackSession {
-    let now = Utc::now().timestamp().max(0) as u64;
-    TimeAttackSession {
-        remaining_secs,
-        wins,
-        saved_at_unix_secs: now,
-    }
-}
-
 /// Inner helper: delete `*.tmp` entries inside `dir`.
 ///
 /// Per-file errors (already deleted, permission denied) are silently ignored.