Shared scheduler fill signal (forward water-marks + hard byte cap) pauses
the C# read loop above high-water and, for Opus, stops the demux/decode
feed so WebCodecs queues stay near-empty. Routes through the existing
cancellation discipline; releases the latch on clear/seek.
Surface the sidecar duration on the first Opus chunk instead of gating it on the first decoded buffers; C# locks UI Duration on chunk 1, and async WebCodecs decode left it at 0 — killing seek and the duration-gated visualizer.
Seek now trims the lead-in so playback lands at the requested time, not the page start; decoder drain polls decodeQueueSize (bounded) instead of a single timeout. Minor cleanups.
daniel-c-harvey