refactor(engine): first ambiguity burn-down batch — 302 → 198 pairs

Two structural fixes from the #143 backlog:

- Game-state ordering spine: the three pre-mutation GameStateResource
  writers (tick_elapsed_time, sync_settings_to_game,
  handle_restore_prompt) are now a deterministic chain before
  GameMutation, and the remaining unordered readers
  (update_selection_hud, handle_hint_button, tick_hint_highlight,
  handle_right_click, snap_cards_on_window_resize,
  sync_pile_marker_visibility, auto_save_game_state) are ordered after
  it. Readers now see the current frame's moves deterministically
  instead of racing the mutators.

- NewGameRequestWriters set: every in-cluster writer of
  NewGameRequestEvent (buttons, modals, mode picker, seed poller,
  restore prompt) is registered in a shared set marked ambiguous with
  itself — writer-vs-writer append order is meaningless since consumers
  drain the whole queue. Out-of-cluster writers (home, challenge,
  time-attack, win-summary, play-by-seed, difficulty, stats plugins)
  can join the set when the test cluster grows.

AMBIGUITY_BASELINE ratchets 302 → 198. Remaining backlog is dominated
by the Sprite (72) / Transform (48) visual-domain cluster, which needs
per-domain set architecture — next batch.

Refs #143

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

solitaire_engine/src/schedule_checks.rs

@@ -38,7 +38,7 @@ mod tests {
     /// ordering edge — add `.before`/`.after` (order matters) or
     /// `.ambiguous_with` (provably order-independent) at the registration
     /// site. When triage lowers the real count, lower this constant too.
-    const AMBIGUITY_BASELINE: usize = 302;
+    const AMBIGUITY_BASELINE: usize = 198;
 
     fn cluster_app() -> App {
         let mut app = App::new();