强制 WebRTC 使用 H.264

[iamsyc]

变更

• Swift 发布端只选择 H.264 和匹配 RTX，H.264 不可用时直接失败。
• relay 只注册 H.264/RTX，拒绝 VP8-only publisher offer，viewer track 固定 H.264。
• 浏览器播放页检查 H.264 能力，校验 answer SDP 和 getStats selected codec。
• 补充 Swift、页面集成和 relay 单测，覆盖 H.264-only、VP8 拒绝、payload type 重写。

原因

• 当前目标是硬性 H.264 链路，删除 VP8 fallback，避免看似协商 H.264 实际落回 VP8。

验证

• swift test --filter WebRTCSessionSupportTests
• swift test --filter WebServerSocketIntegrationTests
• go test -count=1 ./...
• xcodebuild -workspace VoidDisplay.xcworkspace -scheme VoidDisplay -destination "platform=macOS,arch=arm64" build
• Chrome 实测：页面进入直播中，stats selected codec 为 video/H264。
• Safari 实测：页面进入直播中。

[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: 541 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