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/input_plugin/mod.rs

@@ -22,7 +22,7 @@ use std::collections::HashMap;
 use bevy::ecs::system::SystemParam;
 use bevy::input::ButtonInput;
 use bevy::input::touch::{TouchInput, TouchPhase, Touches};
-use bevy::math::{Vec2, Vec3};
+use bevy::math::Vec2;
 use bevy::prelude::*;
 use bevy::window::PrimaryWindow;
 #[cfg(not(target_os = "android"))]
@@ -1806,10 +1806,5 @@ pub fn find_hint(game: &GameState) -> Option<(KlondikePile, KlondikePile)> {
     all_hints(game).into_iter().next()
 }
 
-// `Vec3` is referenced only via the `DRAG_Z` constant; keep the import silenced
-// when the compiler can't see it used.
-#[allow(dead_code)]
-const _VEC3_REFERENCED: Option<Vec3> = None;
-
 #[cfg(test)]
 mod tests;