0.52.2: Polling recovery, safe disconnect and Windows downloads

• Polling recovery: Non-conflict poll/admission failures back off from 1 to 30 seconds and keep retrying, allowing recovery after prolonged outages without manual reconnect. Backoff resets only after durable admission succeeds; persistent competing-client conflicts retain their existing terminal stand-down.
• Disconnect safety: Failed or unavailable Thread cleanup still attempts one captured local transport stop without bypassing Workspace fences. Reconnect revokes stale cleanup/stop continuations; delayed teardown cannot release replacement ownership. Unconfirmed cleanup is reported without claiming it was skipped, while uncertain stop/release outcomes remain incomplete and are never blindly retried (#310).
• Windows downloads: Publishing a complete attachment retries transient Windows EPERM/EACCES sharing failures for at most six rename attempts, without replaying HTTP. Cancellation stops the bounded wait; permanent/non-sharing failures still propagate and remove the private partial file. Concurrent downloads retain deterministic names and complete-file publication.