* refactor: extract remote mount resolution into shared helpers
* refactor: share the adaptive remote cache wait policy
* filer: stream cold remote reads from the origin while caching
* s3: stream cold remote reads from the origin instead of 503 retries
* test: cover the S3 origin stream-through path
* remote mounts: match on path components and prefer the longest mount
* fail short origin streams instead of silently truncating
* s3: try the origin before failing a cold read on a local cache error
* s3: gate origin streaming on the entry's resolved version
* return the cache RPC's NotFound as a canonical status and classify it everywhere
* filer: keep multipart-range cold reads on the retry path