N1: Delete unused CodecSelector.selectEncoder. VideoCompressor calls
findEncoder directly; selectEncoder was dead since the initial commit.
N2: Switch Android output filename from System.currentTimeMillis() to
UUID.randomUUID(). Eliminates the collision risk when two compressions
start in the same millisecond.
N3: Guard CheckedContinuation against double-resume in both
processVideoTrack and processAudioTrack. AVAssetWriter shouldn't
re-invoke the requestMediaDataWhenReady block after markAsFinished, but
if it did, the previous code would crash via CheckedContinuation's
double-resume detection. Added a `finished` flag plus a local `finish`
closure that no-ops on second call.
N4: DataRateLimits window duration changed from Int `1` to Double `1.0`.
VideoToolbox expects the duration value as a CFNumber with float
semantics; Int bridges to NSNumber(int) which works on most iOS
versions but is not spec-correct.
vineyardbovines
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