feat(screenshare): drain a lagging viewer back to the live edge

On a lossy link the reliable PixelPass transport turns every loss burst
into buffered latency that nothing trims back, so the viewer settles
seconds behind the host and stays there. Measured on a tc netem satellite
simulation: a viewer parks at a ~6 s standing buffer indefinitely.

--untimed (0.6.5) does NOT fix this and measured marginally worse (+1.38 s
vs +1.24 s): it only unpaces presentation, while audio still drains at 1x
the DAC rate, so an accumulated backlog never shrinks. Drop it.

Instead give mpv a JSON IPC socket in the Low latency posture and drive
playback slightly fast while the buffer is deep, returning to 1x once it
drains. Pitch correction keeps it inaudible and A/V sync is preserved,
because audio and video speed up together.

The control law and IPC message handling are pure functions with unit
tests; the only I/O is livesync::drive, which ends by itself when the
player exits. Smooth is deliberately excluded — its ~2 s readahead is the
point of that posture, and catch-up would fight it every poll.

Known limitation: 1.05x needs ~120 s to clear a 6 s backlog, so recovery
is slower than ideal. Tuning (a proportional law, or a seek-to-live for
large backlogs) is the follow-up.

598 lib tests green (+11), clippy clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -4,6 +4,16 @@ All notable changes to PeerSpeak are documented here.
 
 ## [Unreleased]
 
+### Fixed
+- **A screen share that falls behind now catches back up.** On a lossy
+  connection (satellite links are the worst case) the share could settle several
+  seconds behind the host and simply stay there for the rest of the call. The
+  viewer now notices a deep buffer and plays imperceptibly fast until it is back
+  at the live edge — the audio stays in tune and in sync while it does. This
+  replaces the previous attempt at the problem, which measurement showed did not
+  help. Applies to the Low latency setting; Smooth intentionally keeps its
+  larger buffer.
+
 ## [0.6.5] — 2026-07-19
 
 ### Added