fix(desktop): recover stale transcript subscriptions

[testikun]

Summary

• Treat only session.transcript.page + not_found as a recoverable subscription failure.
• Reuse the existing subscription replacement and snapshot/transcript reconciliation path.
• Add a deterministic observer regression test proving that live frames continue after the stale subscription is replaced.
• Keep other not_found operations and session_removed terminal.

Fixes #3748

Verification

• Desktop typecheck passes.
• Focused Runtime Host session observer suite passes (37 tests).
• Full Desktop main-process suite passes (1,476 tests).

简体中文摘要

当旧 Session subscription 在 transcript 分页期间失效时，Runtime Host 返回的是 RuntimeHostOperationError("session.transcript.page", "not_found")。Desktop 之前没有把它识别为可恢复错误，导致后台任务继续运行但界面停止接收实时更新。

本 PR 只放行这一组精确错误，复用现有的重新订阅、snapshot/transcript reconcile 和后续 frame 处理流程；其他 not_found 和 session_removed 行为保持不变，并增加确定性回归测试。

[Astro-Han]

I reviewed this head and found no blocking issues.

The fix correctly narrows the not_found predicate to only operation=session.transcript.page — the sole exact pair emitted when an active transcript subscription is reaped. Other not_found failures remain terminal. The linearisation (detach/close failed attempt, open new subscription, recover via snapshot/projector reconcile) is sound, and the new test covers the page-not-found window without requiring renderer resubscribe.

Checks on 97105e7d3c are test: in_progress — code is GO.

简体中文

该头未发现阻断。

[Astro-Han]

APPROVE — 97105e7, no P0-P2, replacement seam correctly scoped.