feat(game): cancellable detector service

Step 4 of game detection. One std-thread worker owns the SteamProbe cache
+ Debouncer across ticks, polls the OS adapters every 3s off the async
runtime, and publishes the stable detected game on a tokio watch channel
only when it changes. Manual override + process map are live-updatable via
shared handles; a cancellable sleep honors stop promptly; drop stops it.

The per-tick decision (match + resolve + debounce) is the pure poll_once,
unit-tested with synthetic Steam/process inputs (debounce, process-only
match, immediate manual override). +4 tests (397 lib).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/game/mod.rs

@@ -12,6 +12,7 @@
 //! ([`scan`]) — feed already-parsed values into these pure functions, and the
 //! cancellable poll service ([`detector`]) wires them together.
 
+pub mod detector;
 pub mod scan;
 pub mod steam;
 pub mod vdf;