feat: support validated same-session conversation rewind

[Wolf-L]

Behavior and integration

Adds negotiated same-session conversation rewind without creating another Codex thread. _session/rewind validates the user-message identity, fingerprint/occurrence and retained assistant boundary, then verifies the persisted same-ID prefix after native revert. Legacy rollback is used only for an explicitly legacy history mode. Ambiguous writes are not retried; successful load is required to reconcile an unknown history outcome. This does not restore files.

Replaces #596 after withdrawing the earlier submission. Refreshed on upstream main 202e66e63343ec4f5a5e35ab590251d813d129ca (2.2.2), retaining custom instructions, AIR sessionIndex, title selection and app-server recovery. The contract and original implementation credit Nikita Ashikhmin's #508; the refreshed history consolidates the old integration.

New regressions cover crash recovery before rewind, no replay after a lost native acknowledgement, index mutation admission, provider-update/close ordering, idle title invalidation and dispatched-title draining across load/resume/close. A retired title generator cannot restart work; unknown title acknowledgements remain fenced. The upstream bounded title wait during load is preserved.

The full companion #611 adds runtime/discovery/archive/attachments and previewed file reversal. It contains this core scope; review/merge core first and refresh the companion afterwards. The persistent queue proposal is independent.

Local validation

• Ubuntu 24.04 / Node 24.14.0: typecheck/build passed; full suite 1546 passed, 36 skipped (single worker).
• Windows targeted rewind/title/recovery regressions: 68 passed. The broader recovery/index run has the same five subscription-notification failures as untouched main; no all-green Windows full-suite claim.
• Pinned native Codex 0.160.1 with isolated home/workspace and a scripted loopback provider: 13/13 on both Windows and Linux. Historical/latest/first/repeated-message rewind, cold reload, resend context, interruption and child shutdown are checked.
• All six Bun release targets compile. Cross-compilation is not physical ARM/macOS runtime validation.
• No real account/model calls, installed package changes or user sessions were used. Local results are distinct from hosted CI and maintainer approval. The loopback/deny-proxy environment is not an OS firewall.

中文

撤回旧 #596 后，按最新主线 202e66e（2.2.2）重新提交同 ID 持久对话回退。保留新上游自定义指令、sessionIndex、标题选择和 app-server 恢复；先验证身份与保留边界，再执行并核对原生历史，未知结果不自动重试。协议及原始实现注明 #508 作者归属。

补齐崩溃恢复、索引修改互斥、provider 更新与 close 的取消顺序，以及跨 load/resume/close 旧标题任务的失效/回执保护。回退不恢复文件；完整增强 companion 包含本核心，应先审核心；原生持久队列为独立提案。

本地 Linux 全量 1546 通过/36 跳过，类型检查和构建通过；Windows 定向回归通过；双平台隔离原生各 13/13，六个平台交叉编译通过。Windows 的五项上游基线失败如实保留，未声称全套全绿或真实账号/模型/ARM 实机验证。托管 CI 与维护者批准另行判断。

Extension-only review after the full companion is published: core → extensions.