Mac in the headset: end the Frame's viewer browser after Stop; relay fixes

- Chromium on the Frame outlived its last viewer window (verified: 11
  processes left after a run). Once nothing is shown, Stop ends it, unless
  Show was pressed again meanwhile; its profile is Frame Control's own.
- Relay --delay: if the agent side fails, close the viewer side too
  (review round 9).
- Docs: the final scroll run captured 57 fps; don't blame ScreenCaptureKit.

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

docs/mac-in-headset.md

@@ -204,7 +204,7 @@ about 5 ms to scroll compared with the baseline's way of measuring:
 | Scenario | Content | Input to drawn | fps drawn | Grades |
 |---|---|---|---|---|
 | test | 10.5 / 16.7 | 29.6 / 36.3 | 60 | all within target |
-| scroll | 19.8 / 27.4 | – | 55.9 | fps, late frames (4.8%) and worst gap (222 ms) only "acceptable": Wi-Fi stalls and ScreenCaptureKit's 46–53 fps from virtual displays |
+| scroll | 19.8 / 27.4 | – | 55.9 | fps, late frames (4.8%) and worst gap (222 ms) only "acceptable": Wi-Fi stalls (the Mac captured 57 fps in this run; earlier runs got 46–53 from virtual displays) |
 | type | 15.0 / 21.4 | 43.0 / 60.5 | – | all within target |
 
 Of the targets, click to photon is met without the Frame's compositor (the