fix(windows): gate active-theme writes with a short update marker

[starsstreaming]

Summary / 摘要

Stacked on #290 (feat/video-backgrounds).

Gate Windows active-theme commits with a short-lived .theme-update-in-progress marker so the injector watcher cannot load a mixed image/video/json/css stamp mid-write. Large files are prepared first; the marker covers only the final commit window and is cleared on success. Stale markers expire so a crashed writer cannot freeze theme loads forever.

叠在 #290 之上。Windows 活动主题提交增加短窗口 .theme-update-in-progress 标记，避免观察者读到半套文件；大文件先备妥，标记只罩最终提交瞬间，成功后清除；过期标记不永久卡住加载。

Depends on / 依赖: #290 — please merge that first (or merge this only after #290). Base branch for review: starsstreaming:feat/video-backgrounds.

Type / 类型

• Bug fix / 缺陷修复
• Feature / 新功能
• Docs / 文档
• Theme / CSS / visual / 主题或视觉
• Scripts / install / restore / 脚本或安装恢复
• Chore / 杂项

Platform / 平台

• macOS
• Windows
• Both / 双平台
• Docs / repo only / 仅文档或仓库元数据

Self-check / 自测

Windows

• Relevant scripts / tests exercised / 已跑相关脚本与测试
• Environment noted below / 下方注明环境

User-facing

• Updated windows/CHANGELOG.md under ## 未发布 (no VERSION bump)

Security / 安全

• Does not modify official Codex install / asar / signatures
• Does not silently write API Base URL or keys
• CDP remains loopback-oriented (127.0.0.1)

Notes / 补充

Behavior

• Set-DreamSkinActiveTheme writes marker token around final moves of image/video + theme.json / css
• loadTheme rejects while marker is fresh; end-of-load also fails if marker appears mid-read
• Watcher waits for commit on startup; audit loop skips reload while themeUpdatePending
• Stale marker (>120s) ignored

Checks

• node --test windows/tests/injector-bootstrap.test.mjs — pass
• Stack tip also covered by full windows/tests/run-tests.ps1 on the next PR (verified rollback) — exit 0
• git diff --check clean on commit

Stack

1. feat(runtime): add cross-platform MP4 video backgrounds #290 video backgrounds ← base
2. this PR atomic theme write marker
3. verified tray rollback (next)
4. CSS narrow native surfaces (last)

Environment

• Windows 11 Pro 10.0.26200
• Branch: starsstreaming:feat/atomic-theme-writes @ 7702162

[starsstreaming]

Stack: depends on #290. Next: #292 → #293.

[Fei-Away]

维护者复核：这份 PR 作为提交形态已不再可合并，因此关闭；atomic-write 意图本身仍有价值。

该分支声明依赖 #290，实际文件列表也携带完整视频栈与数十个无关文件，不能把两行级别的 active-theme 提交门禁独立审阅。当前 v1.5.12 已有导入替换 journal/commit marker、operation mutex、社区应用 snapshot/visible verify/rollback；watcher 对读取中的非法组合会保留旧 payload 并稍后重试。不过普通 tray 写入仍可单独评估更短的发布边界。

如继续，请只从 current main 提交不依赖视频 schema 的 Windows active-theme 原子发布补丁，并补 watcher 在写入中途不会装载混合 revision 的测试。未删除分支，未合并代码。