useMediaURL

Resolve the lazy MediaBase.objectURL promise into a reactive string suitable for <audio src>, <img src>, <video src>, or <a download href>. Returns undefined until the URL is available.

Lifecycle:

• On setup (or whenever the supplied media value changes), the composable awaits media.objectURL and commits the resolved string to state.
• On scope disposal (or when media changes), the composable calls media.revoke() to free the blob URL slot. The next consumer that accesses media.objectURL mints a fresh URL from the same Blob, so live re-renders just work.
• If the underlying handle errored before settling, the URL stays undefined. Surface the failure via media.error.

media accepts a raw handle, a Ref<MediaBase | undefined>, or a getter, so the composable can rebind automatically to the latest media without a manual watcher at the call site.