_hasRenderedFirstFrame had five writer files with hand-rolled
snapshot/restore pairs in the reload and channel-zap rollbacks, and the
paired _hasFirstFrame ValueNotifier was reset independently along the
open path. Move both flags into FirstFrameGate with explicit verbs —
markReady, resetUiForOpen, resetRenderedForAttempt, reset,
forceUiReadyOnFailure, snapshot/restore — so the UI-vs-reporting
asymmetry and the transactional rollbacks are enforced by one type.
The uiReady notifier keeps its identity across attempts; the video
surface, controls, and buffering overlay hold it by reference.
Unit-tests the gate's verb semantics.
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