[Octobersama]

EX 难度页隐藏后，ResizeObserver 会读到零高度，使返回转场捕获到 24px 黑带终态。现在忽略隐藏页的零高度，并在返回目标页恢复布局后同步测量，再捕获动画终态。

补充真实 Extra Start → EX → 自机完整入场 → 返回的回归，检查转场终态与最终 EX 黑带均等于文案高度加 24px。同步 AGENTS，版本号由提交钩子更新。

验证：语法检查、114 项逻辑测试、36 项完整 E2E 通过（受监督进程退出码 0）。桌面和手机实测终态／最终高度分别为 251px、153px。git diff --check 通过。

后续过渡页计划不在此次范围。用户授权通过门禁后合并。

本次改动修复从 EX 难度页返回时，黑带因隐藏页面的零高度测量而先变细再恢复的问题。js/ui.js 观察 EX 按钮尺寸，仅在测量高度大于零时更新黑带；返回转场期间，目标页激活后会同步测量，再捕获转场终态。

新增端到端回归测试，验证自机页完成入场后返回 EX 时，转场终态和静止黑带均与 EX 文案高度加 24px 相符。另更新 AGENTS.md 的尺寸测量说明，并将版本号从 186 更新为 187。

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📝 Walkthrough

Walkthrough

此改动更新 Extra 难度焦点带的尺寸测量，并在返回难度页时重新测量。新增窄屏端到端测试，检查转场终点和动画结束后的焦点带高度。构建号从 186 更新为 187。

Changes

Extra 难度焦点带

Layer / File(s)	Summary
焦点带尺寸测量 js/ui.js	_rebuildDifficulty 将尺寸观察逻辑委托给 _fitExtraBand。该方法在测量高度大于零时，将 Extra 按钮高度向上取整并加 24px，设置 --extra-band-height。
返回转场和验证 js/ui.js, test/e2e/menu.spec.js, AGENTS.md, js/version.js	转场目标为难度页时，重新计算 Extra 焦点带。文档说明隐藏页零高度的处理和目标页激活后的测量。新增测试验证转场终点及动画结束后的高度。构建号更新为 187。

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 75453

返回 EX 难度页时会在布局恢复后更新焦点带高度，再生成转场终态，未发现阻止合并的风险。

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	标题清楚概括了主要改动：修复从 EX 难度返回自机选择页时黑带先变细后恢复的问题。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 3…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
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