feat(game): Steam + process-scan OS adapters

Step 2-3 of game detection (game-presence-plan.md). The OS edges feeding
the pure seams from the previous commit.

- src/game/steam.rs: SteamProbe — reads the live RunningAppID and resolves
  it to a name via appmanifest_<id>.acf (no binary appinfo.vdf). Pure parse
  fns (parse_running_app_id / parse_library_paths / parse_app_name) over
  file contents are unit-tested incl. current+legacy libraryfolders shapes,
  escaped Windows paths, empty/missing names, and garbage. Roots discovered
  across native/Flatpak/Snap (Linux) and the registry (Windows); libraries
  and resolved names cached + mtime-invalidated so the 3s poll doesn't
  rescan. File reads byte-capped.
- src/game/scan.rs: native running-process enumeration — /proc (exe symlink,
  comm fallback) on Linux, Toolhelp on Windows — feeding the pure
  match_processes. No sysinfo dep (D7).
- Cargo.toml: windows-sys as a direct Windows-only dep for the registry +
  Toolhelp FFI. No NEW crate — it was already in the lockfile transitively
  via cpal/rfd, so the audit surface is unchanged.

391 lib tests (+5). Linux: build + clippy --all-targets clean. Windows FFI
signatures verified against windows-sys 0.61 source (one *const vs *mut
lpReserved fixed) but NOT yet cross-compiled — defer to the post-UI Windows
build cycle.

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

Cargo.lock

@@ -4894,6 +4894,7 @@ dependencies = [
  "thiserror 2.0.18",
  "tokio",
  "tokio-stream",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]