### Change summary
Defer rollout file creation until needed.
* Add a core API to force rollout persistence for loaded non-ephemeral threads:
* seeds initial context if needed
* flushes rollout and returns persisted path
Add concurrency guard to make lazy rollout initialization idempotent under concurrent calls.
Add centralized app-server rollout-path resolver that:
* uses in-memory thread state when loaded
* forces persistence on demand for rollout-dependent calls
* falls back to on-disk lookup for unloaded threads
* maps ephemeral threads to invalid-request errors for rollout-dependent operations
Route rollout-dependent endpoints through the resolver (v2 + shared legacy surfaces), including:
* thread/archive
* thread/resume (thread-id path)
* thread/fork (thread-id path)
* resumeConversation
* forkConversation
* thread summary by thread id
* detached review parent-thread path resolution
* feedback include_logs rollout resolution
Remove stale cached rollout-path assumptions in rollback/detached-review flows by resolving via thread id when needed.
No wire-schema changes; behavior-only change.
v1 compatibility is not expanded in this PR.
### Tests updated/added
* thread_start: assert rollout is absent immediately after thread/start; created after first completed turn.
* thread_resume: resume by thread id succeeds for just-started thread via on-demand persistence; path-vs-thread-id precedence test updated.
* thread_fork: fork by thread id succeeds for just-started thread.
* thread_archive: archive succeeds for just-started thread and materializes before archive.
* thread_unarchive: adjusted for deferred creation timing.
* thread_rollback: rollback path no longer depends on stale cached rollout path.
* Detached review targeted test verified for lazy path behavior.
* Core tests for new persistence API
