headless=True now hides the window via the binary's own cloak pref
(zoom.stealth.cloak_windows) on Windows and macOS instead of the broken
thread-level SetThreadDesktop; macOS is now supported. Linux keeps Xvfb.
Adds e2e guards that also run per-platform in the release drive-gate:
- test_cloak: the window is hidden (Windows DWMWA_CLOAKED / macOS CGWindowAlpha)
yet still renders + drives; the macOS leg is where the cocoa cloak patch runs.
- a WebGL readPixels masking guard: the gamma noise must stay a smooth gamma
remap, not the pixelscan-maskable +-1 spikes.
feder-cr