Skip the back-pressure interop poll while paused (UC5). Document complete()
draining the stash in full by design. Rename scheduler isProductionPaused to
evaluateProductionPause (latch-advancing); window exposure name unchanged.
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.
The inclusive <= bound is correct; comments now say 'at or behind'. New
test drives eviction through the real onended trigger with a live mid-array
source pinning the frontier.
Drop already-played buffers from the front while advancing the time
anchor so position/index bookkeeping stays exact. Shared by both decode
paths, no format branch. Back-retain is a config seam for 21.2.
daniel-c-harvey