Mac in the headset: stream Mac windows and screens into the Frame as panels

Frame Control can now show any Mac window, or a whole screen, as its own
SteamVR panel on the Steam Frame (Tools -> Mac in the headset, macOS only).
Place it with the SteamVR dashboard; the laser clicks and scrolls, and the
Mac's own keyboard types.

- mac/frame-mac-view (Swift, no dependencies): ScreenCaptureKit capture per
  window or display, VideoToolbox H.264 with low-latency rate control (JPEG
  fallback), a loopback HTTP/WebSocket server, CGEvent/AX input playback,
  and a display-awake assertion while anyone watches.
- ui/frame_macview.py: starts the agent, runs an ssh -R tunnel with a
  supervisor that reopens it on the same port, and launches a Chromium app
  window per stream on gamescope's :0, tagged with STEAM_GAME for its own panel.
- Frame Control's key never leaves the Mac: viewers get single-use,
  per-source tickets and reconnect keys that Stop revokes.
- ui/mac-view.html: WebCodecs decode, keyframe recovery, pointer/wheel/keys back.
- Bundled in the Mac app build; tests/test_macview.py builds and drives the
  agent on macOS.

Verified on the Frame (build 20260925.6191901) with the test pattern: panel
in about 1.5 s, about 60 fps, Mac-to-window about 11-17 ms, tunnel recovery
in 4 s. Laser input and real window capture still need a person in the headset.

Also commits the other thread's first-party rule (steam-frame skill) and
the first-party options table in docs/streaming.md.

Reviewed by GPT-6 Astra (xhigh, read-only) over six rounds; all findings fixed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/frame-control.md

@@ -70,6 +70,10 @@ counts them while they run.
   Runtime picked from the program's header), listed under **Sideloaded titles**
   with Launch and Remove; see [sideloading.md](sideloading.md). Send typed text, or your computer's clipboard, to the
   Frame clipboard.
+- **Mac in the headset** (macOS): show any Mac window, or a whole screen, as
+  its own panel in the headset. Place it with the SteamVR dashboard, click and
+  scroll with the laser, and type on the Mac. Streams hardware H.264 through
+  an SSH tunnel; see [mac-in-headset.md](mac-in-headset.md).
 - **Flatpaks**: install and remove them (quick picks: Moonlight, Firefox, VLC,
   Remmina).
 - **One-click tools**: SSH or SFTP in a terminal window, Steam Link, and remote