useMediaURL

Resolve the lazy MediaBase.objectURL promise into a reactive string suitable for <audio src>, <img src>, <video src>, or <a download href>. Reads .current for the latest URL, or undefined until the promise settles.

Lifecycle:

• On first $effect (or whenever the supplied media value changes), awaits media.objectURL and commits the resolved string to state.
• On cleanup (or when media changes), 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 or a getter so the composable rebinds automatically to the latest media without a manual effect at the call site.