[codex] Rebuild Web sharing around AV1-only transport

[iamsyc]

Summary

This PR rebuilds the Web sharing path around AV1-only video transport.

It updates the macOS publisher, relay, and browser viewer so the production WebRTC path negotiates AV1 only, rejects unsupported video codecs, and keeps the relay as an RTP forwarding layer without transcoding. It also makes the streaming profile source-spec aware so Automatic and Smooth modes target the source display resolution while exposing actual runtime stats in the publisher, relay, and browser viewer.

Changes

• Constrain publisher codec preferences to AV1 and fail clearly when the WebRTC stack does not expose AV1 video encoding.
• Register and forward AV1-only video in the relay, with negative handling for non-AV1 publisher and viewer offers.
• Update browser receiver preferences, SDP answer checks, and stats confirmation for AV1-only playback.
• Add source video spec handling for capture dimensions and refresh-rate-derived frame targets.
• Add codec demand, relay selected-codec visibility, and publisher/browser diagnostics for encoded and decoded video stats.
• Update app-facing copy to describe AV1 source-resolution Web sharing.
• Expand Swift and Go tests for codec filtering, relay negotiation, RTP forwarding, browser page behavior, and parent-process relay shutdown.

Verification

• swift test
• go test -count=1 ./... in Tools/VoidDisplayRelay
• xcodebuild -project Apps/VoidDisplay/VoidDisplay.xcodeproj -scheme VoidDisplay -destination 'platform=macOS' build
• codesign --verify --deep --strict --verbose=2 on the built Debug app
• Chrome real playback check: AV1 at 3840×2160
• Safari real playback check: AV1 at 3840×2160
• Relay snapshot check: publisher and subscribers use AV1, with zero drops in the final run

[github-actions]

CI Summary

Run: Open

Check	Status	Details
Change Scope	ui_code	code=true, ui=true
Dependency Review	success	code PR gate; high and critical vulnerabilities block
Static Checks	success	actionlint, shellcheck, shfmt, SwiftFormat, SwiftLint, action pinning
Unit Tests	success	Swift tests: 547 Go packages: 2 Reason: passed Artifact: unit-gate
Xcode Build	success	Debug build with zero warning scan Artifact: xcode-build
UI Smoke	success	Failure classification is written by ui-smoke-summary.json in each artifact. Artifacts: ui-smoke-baseline, ui-smoke-permissionDenied, ui-smoke-rebuildFailed
Release Smoke	success	Artifacts: release-smoke-arm64, release-smoke-intel64 Release verify summaries are produced by release.yml before publishing.
Artifacts	link	Open artifacts
CI Gate	success	single branch protection check