feat(flow): on-demand host handoff with reliable return and a back-tap tip

Keyboard
- Only jump to the app when the voice surface is shown (tab tap / opened
  there); typing tabs no longer leave the host on keyboard open.
- Wait up to 0.4 s for the arbiter to report the host app before leaving,
  and late-bind an id that arrives within 6 s of the jump.
- Empty arbiter callbacks no longer wipe an already captured host id;
  install the capture handler in every build configuration.

App
- Return to the previous app with up to four retries using iOS's real
  open result, and honor the skip-app-switch preference.
- Keep the pending host id across cold-start zombie reconciliation, and
  expire it after two minutes (legacy values without a timestamp stay valid).
- Show a light tip pointing at the system back control when we cannot
  return automatically ("点击返回，立即使用").

Tests: TTL behaviour and return retry policy.

CHANGELOG.md

@@ -7,7 +7,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- **“Tap back” tip when we cannot return for you**: after the keyboard sends you to OSG to start voice, and OSG cannot jump back on its own (the app you came from is unknown, or the jump failed), a light veil appears with a small arrow pointing at iOS's own back control at the top-left and one line, “点击返回，立即使用” / “Tap back, start right away”. It works even when OSG could not tell which app you were in, because the system control already names it. Tap anywhere to dismiss; it also disappears as soon as you leave. / **无法替你返回时的「点击返回」提示**：键盘带你进入 OSG 启动语音后，如果 OSG 没能自动跳回（不知道来自哪个 App，或跳转失败），会出现一层淡淡的遮罩，一个指向 iOS 左上角系统返回按钮的小箭头，以及一句「点击返回，立即使用」。即使没识别出你来自哪个 App 也会显示，因为系统按钮本身就写着来源 App 的名字。点任意处关闭，你一离开也会自动消失。
+
+### Changed
+- **The keyboard only opens OSG when you need it**: opening the keyboard on the typing tabs (拼音 / English) no longer jumps to OSG. The jump now happens when you land on the voice tab, or when you start dictation, ask AI, or run a clipboard skill (including automatic ones). A short cool-down keeps flipping between tabs from reopening the app. / **只在需要时才打开 OSG**：在拼音 / 英文标签页打开键盘不再跳转到 OSG。切到语音标签页，或开始听写、提问 AI、使用剪贴板技能（包括自动触发的）时才会跳转；短暂的冷却时间避免来回切换标签页反复拉起 App。
+- **Auto-return now retries and respects your setting**: going back to the previous app is tried up to four times, waiting for voice to be ready and for a late app identification, instead of a single attempt, and it now follows the “return to previous app after starting” preference instead of ignoring it. / **自动返回会重试，并遵循你的设置**：返回上一个 App 最多尝试四次，会等待语音就绪和迟到的 App 识别，不再只试一次；同时遵循「启动后返回上一个 App」设置，而不是忽略它。
+
 ### Fixed
+- **Host app identification no longer flickers**: the keyboard waits up to 0.4 s for the system to report which app you are in before it leaves, keeps a correct identification when a later empty system callback arrives, and still records an identification that arrives a few seconds after the jump. / **宿主 App 识别不再忽有忽无**：键盘离开前最多等待 0.4 秒，让系统报告你所在的 App；之后到达的空回调不会再抹掉已识别到的结果；跳转后几秒内才到达的识别结果也会被补记。
+- **Return target survives a cold launch and expires safely**: starting OSG after it had been force-quit no longer wipes the app to return to, and a recorded target older than two minutes is ignored so you are never sent to an app you left long ago. / **返回目标不再在冷启动时丢失，也不会过期误跳**：强杀后冷启动 OSG 不再清掉要返回的 App；记录超过两分钟的返回目标会被忽略，不会把你送回早已离开的 App。
 - **Auto-return to the previous app in every build**: host-app identification used to compile only in Debug, so TestFlight and App Store builds never knew which app you were typing in and could not jump back after starting a voice session. It now runs in Debug, TestFlight, and Release; if the system does not expose the host (iOS earlier than 26.4), OSG simply stays open as before. / **所有版本都能自动跳回原 App**：此前识别宿主 App 的逻辑只在 Debug 构建中编译，TestFlight 和 App Store 版本不知道你正在哪个 App 里输入，启动语音会话后无法自动跳回。现在 Debug、TestFlight、正式版都会识别；系统不提供宿主信息时（低于 iOS 26.4）与之前一样停留在 OSG。
 
 ## [2.1.6] - 2026-10-27