test(engine): ratchet on Bevy system-order ambiguities

First measurement of schedule hygiene (issue #143): the headless
gameplay cluster (Game/Table/Card/Hud/AutoComplete/UiModal/UiFocus/
Settings) carries 302 system pairs with conflicting data access and no
ordering edge. Too many to triage in one pass and most are likely
benign event/resource writers — but unproven, and nothing stopped the
count from growing.

New schedule_checks test builds the cluster with ambiguity detection
promoted to error, parses the reported pair count, and asserts it never
exceeds AMBIGUITY_BASELINE (302). New ambiguous pairs now fail CI at
the PR; the legacy backlog can be burned down incrementally by adding
.before/.after or .ambiguous_with and lowering the baseline.

Refs #143

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

solitaire_engine/src/lib.rs

@@ -43,6 +43,7 @@ pub mod replay_overlay;
 pub mod replay_playback;
 pub mod resources;
 pub mod safe_area;
+mod schedule_checks;
 pub mod selection_plugin;
 pub mod settings_plugin;
 pub mod splash_plugin;