GRE-223 production viewer and WHEP session cleanup

[HeadCase]

Implements GRE-223 and the approved GRE-204 performance correction. Integrates the production viewer, fixes exposure-rail and layout instability, keeps the encoder fixed at 1.5 Mbit/s, and adds validated same-origin WHEP session cleanup to prevent departed sessions multiplying WAN traffic. Includes deployed 1/2/4-viewer measurements and full repository quality-gate results in docs/verification/gre-223-production-viewer.md. The checked-in browser suite was typechecked; affected deployed paths were exercised directly through the operator-Mac Playwright connection.

[HeadCase]

Final deployed Playwright acceptance completed against the exact release on the real ASI662MC over 10.44.0.1. One viewer: Live, decoded video advanced 1.201 s over 1.2 s, command width remained 736 px, panel/draft geometry delta 0 px, panel centre delta 0 px, attachment gap 6.4 px, colour and mono both reached exactly visible, and the 2 s rail advanced monotonically from 1.6% to 49.5% with no regressions. Settings restored to 50 ms / ISO 600 / B&W; no viewer-console errors. Four viewers: all Live, all advanced 1.20-1.25 s, all 736 px command bars, all 12 detents. After departure MediaMTX reported paths_readers=0 and webrtc_sessions=0. Mobile 430x932: no overflow or pairwise control overlap, 0 px centre delta, 6.4 px panel gap. Capture, encoder, and relay remain ready with zero camera restarts.