Render a synchronized-output frame on close, not on the next debounce

[s-celles]

Fixes #6071.

Problem

Under a continuous full-screen animation that wraps every frame in a DEC
private mode 2026 (synchronized output) block, the display is capped at ~1 fps
however fast frames arrive and however idle the renderer is — completed frames
only paint when the 1000 ms synchronized-output timeout expires.

When a 2026 block closes, the input handler requests a refresh.
RenderService.refreshRows flushes the buffered rows but schedules the paint
through the render debouncer (requestAnimationFrame). Under a continuous
animation the next frame opens a new 2026 block before that rAF fires, and
_renderRows skips while synchronized output is on — so the debounced paint is
dropped and the frame only appears on the sync timeout.

Fix

Render synchronously when a synchronized-output buffer was just flushed
(buffered is truthy). At that point sync is already false — the flush
runs from the mode-reset that turned sync off — so the completed frame paints
before the next frame can reopen the mode. No partial-frame tearing, since the
frame is complete by construction.

Testing

Measured against a 30 fps animated-background TUI (TryIt.jl): renders went from
~1/s to the frame-arrival rate, with no tearing.

Assisted-by: AI

[jerch]

Hmm to be honest - I don't like the idea to introduce synchronous rendering. I think a better solution would be to introduce double buffering, where DECRST 2026 triggers a viewport buffer copy to be shown on next requestAnimationFrame (with an upper time limit).

@Tyriar Your thoughts on this?