fix(sharing): 稳定网页观看端启动与诊断

[iamsyc]

What Changed

• Shortened the WebRTC publisher startup path by removing the fixed ICE gathering wait before handing the answer back to the browser.
• Added bounded first-frame and publisher-startup waits so stalled startup paths retry or report codec preparation instead of hanging indefinitely.
• Split the display page into HTML, CSS, messages, and runtime resources.
• Updated viewer diagnostics so low-motion desktop updates are described as change-driven screen updates while real degradation remains tied to resolution loss, packet loss, or dropped frames.
• Added coverage for relay startup timeout behavior, service restart cleanup, socket restart reuse, and display page runtime diagnostics.

Why

Opening the viewer page could feel slow before the first picture appeared, and static desktop scenes made browser WebRTC freezeCount look worse than the actual stream quality. The flow now reaches negotiation sooner, handles missing first frames with bounded retry behavior, and reports static low-motion scenes with a product diagnosis that matches how screen capture emits frames.

Validation

• swift test --filter 'RelaySessionHubTests|WebServerSocketIntegrationTests|WebServiceControllerTests|SharingEndToEndIntegrationTests'
• xcodebuild build -workspace VoidDisplay.xcworkspace -scheme VoidDisplay -configuration Debug -destination 'platform=macOS'
• Scanned .ai-tmp/pr-merge/xcodebuild-debug.log for warning/error patterns: no matches.
• git diff --check

Manual Verification

• Browser viewer reported AV1 3840×2160 source and playback resolution.
• Dynamic motion on the shared display reached about 50.78 decoded fps over a 5-second sample with no dropped frames in the observed interval.
• Static desktop low-fps readings were confirmed to come from change-driven screen updates rather than reduced source resolution.

[github-actions]

CI Summary

Run: Open

Check	Status	Details
Change Scope	ui_code	code=true ui=true script=true product=true test=true ci=true release=true dependency=false tooling=true docs_only=false unknown=false
Required Gates	derived	static=true dependency=false unit=true xcode=true ui=true release=true
Dependency Review	skipped	required=false high and critical vulnerabilities block
Static Checks	success	required=true actionlint, shellcheck, shfmt, SwiftFormat, SwiftLint, action pinning
Unit Tests	success	required=true Swift tests: 552 Go packages: 2 Reason: passed Artifact: unit-gate
Xcode Build	success	required=true Debug build with zero warning scan Artifact: xcode-build
UI Smoke	success	required=true 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	required=true arm64=success intel64=skipped PR release smoke requires arm64 only. Artifacts: release-smoke-arm64 Release verify summaries are produced by release.yml before publishing.
Artifacts	link	Open artifacts
CI Gate	success	single branch protection check