feat(engine): Show solution — auto-play the winning line from the pause menu

New SolutionPlaybackPlugin: the pause modal's 'Show solution' button
resumes the game and requests a solve of the live position via
GameState::winning_line on AsyncComputeTaskPool (never blocks the main
thread; stale results discarded via a move_count snapshot). The line
then plays back one instruction per 0.45 s through the normal
MoveRequestEvent / DrawRequestEvent pipeline — animations, scoring,
undo history and win detection behave as if the player made the moves.
Tableau run counts are decoded against the live state at step time.

Playback cancels on Esc, pause, undo / new-game requests, and any
rejected move (the signature of the player diverging the board).
Toasts cover search start, line found, unwinnable, and budget-
exhausted outcomes.

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

solitaire_engine/src/events.rs

@@ -159,6 +159,12 @@ pub struct DeleteAccountRequestEvent;
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct PauseRequestEvent;
 
+/// Request to solve the current deal and auto-play the winning line.
+/// Fired by the pause menu's "Show solution" button; consumed by
+/// `solution_playback_plugin`.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct ShowSolutionRequestEvent;
+
 /// Request to toggle the help / controls overlay. Fired by the HUD "Help"
 /// button alongside the existing `F1` accelerator so the overlay is
 /// reachable without a keyboard. Consumed by `help_plugin::toggle_help_screen`.