fix(windows): restore switching and reduce skin churn

[chelsen0806]

Windows switching/performance preflight fix

Summary

• Restore in-app theme-library switching on Windows and launch the switcher outside Codex's process tree.
• Honor Codex's data-reduced-motion="true" root attribute as well as the OS media query.
• Ignore skin-owned DOM mutations in the renderer observer so skin injection does not repeatedly trigger itself.
• Bump the public source tree to 2.6.4 and add regression coverage.

Why

On Windows with the published 3.3.0 package, QQ mode measured about 9.9 FPS while native Codex measured about 180 FPS. The renderer also incremented ensureCalls from 73 to 83 in 8 seconds because its own DOM writes retriggered the observer. Windows in-app library switching currently rejects immediately with In-app library switching is currently macOS-only, and a direct restart child is killed with Codex.

Upstream report: #26

The full reproduction data and the 3.3.0-specific top-bar hit-testing issue are in that report.

Important repository mismatch

The public source main is still 6ba0877 / 2.6.3, while release tags v3.2.1 through v3.3.0 point at that same old commit. The published 3.3.0 engine is newer and cannot be reproduced from this branch. Please rebase the applicable parts onto the real 3.3.0 source before merging. The one-line top-bar z-index fix for the published engine is attached to the linked issue; it is not in this branch because the old renderer places the selector differently.

Verification

• powershell -NoProfile -ExecutionPolicy Bypass -File tests/windows/run-tests.ps1
• Result: Windows static and cross-platform tests passed.
• Built Codex-QQ-Skin-Windows-x64-v2.6.4.zip and Codex-QQ-Skin-Windows-x64-v2.6.4-Setup.exe

Artifacts: https://github.com/chelsen0806/Codex-QQ-Skin/releases/tag/v2.6.4-preflight