server: one synchronized message per frame, cells and images together

A frame went out as two messages: the cell diff, then the kitty
placements that belong on top of it. The client wrote each to the
terminal with its own write, and directly on a tty the two land in one
read often enough that nobody saw the gap. Put anything that re-chunks
the stream in between -- a mirror like shellglass, which reads the pty
8KB at a time and writes each read straight through -- and the gap
becomes a screen refresh of its own: the cells where the picture is,
repainted, with no picture on them, and the placement arriving a moment
later to put it back. Every image on screen blinks once per frame, which
is how it was reported: unstable, as if constantly redrawn.

So the two halves travel as one message now, and the whole frame is
wrapped in synchronized output (DEC private mode 2026), which a terminal
buffers and presents as one update. That covers the case the single
message cannot: a frame larger than one read downstream is still split,
and the markers are what stop the halves being presented separately.
Terminals that do not implement 2026 ignore the private mode, so it goes
out unconditionally -- the same thing tmux does, and the reason a redraw
under tmux never showed this.

The live check asserts what the fix is: no image command reaches the
client outside a synchronized update.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CRz3zwZJvBvzsR9k4cj6dd

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