6.56.9 - The served placement names itself

A follow-up to 6.56.8, and the line its retest needs. Take this one rather than 6.56.8: it carries the same fix plus the witness for it.

The served placement names itself (AE#454)

6.56.8 puts a rejoin's placement into the manifest the fresh item loads. A field log could see the engine ARM that placement and could not see whether the playlist actually offered one, or at what depth, which is the difference between "the tag was not served" and "the client ignored it".

The served tag now says so, and the pair is self-checking: the offset the server writes and the position the fresh item reports at readiness are the same number.

#454 placing the fresh item at 81.70s in its own playlist: segment 15 + 4.95s (output 80.27s)
#454 serving #EXT-X-START:TIME-OFFSET=50.143,PRECISE=YES for segment 15 + 4.95s, 24 segment(s) listed from seg6
#454 the playlist already placed this item at its own 50.14s, 0.003s from the 50.15s the rejoin asked for; no correcting seek

Bounded by the arm: off on every build except the ones between a rejoin swap and the item it placed running, so an ordinary live session logs none of it.

swift test: 2404 tests, 328 suites, 0 failures.

Full diff: 6.56.8...6.56.9