warm_cache

Pre-populate the file cache off the event loop.

Also resolves a project root deferred by set_cwd, so the blocking filesystem walk runs in a worker thread instead of on the event loop.

Warmers are scheduled non-exclusively, so quick cwd/cache invalidations can run concurrently. The generation is snapshotted once before the first await and re-checked after each await, so a warmer whose generation has been superseded drops its results instead of overwriting controller state belonging to a newer generation. (Snapshotting again before the second await would defeat the guard: it would match the post-supersession generation and let a stale-root file walk win.)