Steam Input as the primary guest-input path (optional, SDK-gated)

Add SteamInputSource: initializes SteamAPI + Steam Input, loads a bundled action
manifest via SetInputActionManifestFilePath (no partner-backend config needed),
and reads the GameControls action set into CoopPadState -- falling back to XInput
per slot, and to pure XInput if Steam isn't available, so the host always runs.

Enabled automatically when the Steamworks SDK is vendored at
third_party/steamworks_sdk/ (auto-detected by CMake; gitignored and never
committed -- the build is XInput-only without it). Stages steam_api64.dll + the
manifest next to the host and builds coop_steam_input_probe (a console smoke test).

Verified: the probe initializes against the live Steam client and enumerates
controllers; the host degrades gracefully when launched standalone. All 5 tests
pass. Reading actual controller state needs a pad bound through Steam Input for
the running (donor) appid, which XInput otherwise covers.

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

.gitignore

@@ -10,3 +10,7 @@ CMakeUserPresets.json
 x64
 Debug
 Release
+
+# Steamworks SDK: vendored locally, not redistributable -- never commit it.
+/steamworks_sdk_*.zip
+/third_party/steamworks_sdk/