AvatarProvider

Resolves an avatar for a remote participant's host-supplied peerId (passed to SerenadaCore.join). The call UI renders the returned avatar cover-fit, cropped to a circle above the participant's name when their remote video track is off.

Behavior:

• Each peerId is resolved at most once per call and cached for the call's lifetime.

• Called lazily on the first frame the placeholder is needed — silent peers don't trigger a fetch.

• Returning null or throwing falls back to an initials placeholder.

• The call UI never blocks on the resolver; it shows initials immediately and swaps in the avatar when the suspend function returns.