patcher: bundle Goldberg steam_api64.dll stub + steam_appid

Stripping SteamStub DRM with Steamless is necessary but not
sufficient — Rec Room's Plugins/steam_api64.dll still tries to
talk to Steam at runtime and bails when Steam isn't loaded. The
working test installs swap in Goldberg Steam Emulator (~11 MB),
which fakes a logged-in user without needing Steam running.

- DorkNet.ClientMod/steam-stub/ holds the stub DLL + a 7-byte
  steam_appid.txt (Rec Room is Steam app 471710)
- build-patcher-zip.ps1 copies both into the zip and writes a new
  steam_stub block in manifest.json so the launcher knows where to
  install them
- Manifest schema is back-compat: launchers that don't know the
  steam_stub block just ignore it

Without this, the launcher's Steamless step produced a runnable
exe that exited within a second of launch because of the Steam
API failure. With it, the unwrapped exe starts up cleanly.

DorkNet.ClientMod/steam-stub/steam_appid.txt

@@ -0,0 +1 @@
+471710