Materialize for Direct File Access

Goal: Get a stable on-disk path for an object so you can mmap it, seek through it, hand it to a subprocess that takes a path, or read it many times without re-downloading.

When to use this: The object is large, you need random access, or a tool you're driving (ffmpeg, usdcat, ML data loaders) wants a filesystem path rather than a byte stream. Also when you'll read the same object multiple times — materialize fetches once, pins, reuses.