Usage notes
`configured` means the source definition was accepted; `ready` means its
media input completed loading before the operation resolved. Observe the
adapter's source-state map for later lifecycle transitions.
Signature
TimelineMediaSourceOperationResult: { ok: true; sourceId: string; state: "configured" | "ready" } | { error: Error; ok: false; reason: TimelineMediaSourceOperationFailureReason; sourceId: string }