fix(desktop): hold a playback anchor so lyric lines land on the beat

[aodjo]

Lyric lines arrived early on one verse and late on the next.

Players are not obliged to keep the position they publish up to date. MPRIS
has clients assume playback advances linearly and watch for a discontinuity
instead, and several players round the value or leave it standing between
updates. Reprise was adopting every sample as the new truth, which handed
the player's rounding straight to the lyric sheet twice a second.

Now the position advances on the clock from an anchor, and a sample only
replaces it when playback has genuinely moved: a new track, a change of
state, or a jump beyond the drift tolerance. A seek is still picked up on
the very next poll.

Simulated against a player reporting whole-second positions, polled every
500ms, over a 40-line sheet:

	worst error	spread	line jumped backwards
adopting every sample	700ms	700ms	8 times
holding the anchor	200ms	200ms	never

The remaining 200ms is the panel's own 250ms tick, which macOS shares. For a
player that reports precise positions the two are identical, so the change
costs nothing where it was not needed.

Also here:

• Each player is now dated by its own read rather than by the sweep, taking
  the midpoint of the round trip. A shared timestamp taken before the reads
  ran the estimate ahead by however long they took, and an anchor holds that
  error for the length of a track.
• PlaybackPosition.Project splits the unclamped projection out of
  Estimate, because asking whether playback has jumped is not a question
  about the track's length: a length that has not arrived yet clamped the
  comparison to zero and re-anchored on every poll.
• The active-session tie-break no longer ranks on how recently a session was
  observed. Every candidate comes from one sweep, so that key now only
  favoured whichever player answered last, taking the choice away from the
  player id that keeps the panel from flickering.
• Players are polled every 500ms rather than every second, matching macOS.

https://claude.ai/code/session_0164R7Zuagx737iKYbuuSjnA

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