fix(windows): fail verification on horizontal overflow

[rwang23]

Summary / 摘要

• Fail Windows renderer verification when the document has horizontal overflow.
• Preserve normal vertical scrolling for long conversations.

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 (when code under windows/ changes)

• Relevant install / start / verify / restore scripts exercised / 已按改动跑过对应脚本
• Environment noted below / 下方注明环境

User-facing / 用户可见变更

• Updated windows/CHANGELOG.md / 已更新 changelog
• N/A — no user-facing change / 无用户可见变更

Security / 安全

• Does not modify official Codex install / asar / signatures / 未修改官方安装与签名
• Does not silently write API Base URL or keys / 未静默写入 API Base URL 或 Key
• CDP remains loopback-oriented (127.0.0.1) where applicable / CDP 仍仅本机回环（如适用）

Notes / 补充

Windows already computed documentOverflow.x, but the final pass gate ignored
it. The new fixture keeps scroll axes independent and proves that horizontal
overflow fails while vertical-only overflow passes.

Validated on Windows at main@611c101 with:

• node --check windows/scripts/injector.mjs — passed
• node --test windows/tests/injector-window-readiness.test.mjs — passed
• powershell.exe -NoProfile -ExecutionPolicy RemoteSigned -File windows/tests/run-tests.ps1 — passed
• git diff --check — passed

No live installer, launcher, or Codex verification was performed.

Tracking issue / 追踪 Issue

Closes #298