Prune can race with an in-flight render of a replaced file #1

When a source file changes while a request still holds the old library.Item, render.Prune (run after the rescan) can remove the old version directory while Renderer.generate is writing into it:

• os.Rename fails with ENOENT → that request returns 500.
• or MkdirAll recreates the old directory after the prune → orphan until the next rescan.
• or ServeFile 404s after Get returned.

Self-healing on the next request. Possible fix: prune only version directories older than the previous snapshot, or skip directories with a .tmp-* file younger than a minute.