fix(telemetry): meter both bitrate fields over the media played, not the transfer (#514)

[superuser404notfound]

Round two of #514, from the reporter's retest on Sodalite 1.0.0 (41).

Cause

Both instantBitrateMbps and averageBitrateMbps were metered from demuxerBytesFetched, the reader's transfer counter. Round one fixed the average's divisor; the numerator was the other half of the same defect. Transfer and playback part ways on every route that reads ahead:

• VOD: prefetch and seek re-fetches count the moment they arrive, so the average reports the prefetch.
• Live, paused: the pump keeps draining the origin into the DVR window by design (AE#443), the divisor stands still, the average climbs for as long as the pause runs. The instant field keeps reading the incoming stream.

Fix

• PlayedMediaLedger: the pumps record the played video and audio packets by presentation time on the source axis (native producer next to the recording tap, software host after its timeline fold, both loops). Exact duplicates are held once, a re-read from far behind drops the stale lead.
• PlayedBitrateMeter: each sampler tick charges the packets the playhead crossed since the last one, over the media seconds it crossed. A pause does not move the playhead, a seek (a step no playback rate can explain) charges nothing, and a span with no recorded bytes stays nil instead of reading zero.
• Playhead on the ledger's axis: AVPlayer's clock plus playlistShiftSeconds on native (not sourceTime, which is the item axis on a sequential origin), the synchronizer's source clock on software.
• The transfer stays where it belongs: networkThroughputMbps and [LagDiag]. The remote-HLS bypass keeps its declared rates.

Verification

aetherctl play over a threaded HTTP origin, main against this branch:

Case	main	this branch	truth
Native VOD, H.264, pause 12 s, seek 15 s	avg 723, 103, 52 Mbps	18.4 to 19.0	18.2
Software VOD, VP9, same drill	avg 385, 54, 26	8.8 to 9.5	8.9
Native live TS, DVR 300 s, paused 50 s	avg climbs 23 to 64 during the pause	holds 8.12, then 8.0	8.17
Software live, same	avg climbs 29 to 71	holds 8.07, then 8.0	8.17

swift test: 646 XCTest (1 skipped) and 3584 swift-testing tests green. tvOS build green. Issue514AverageBitrateTests now covers the reporter's four cases, seek plus re-fetch, and an off-axis playhead.

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