…not just content

The terminal-story readiness gate waited only for text to reach the xterm
buffer, which happens almost instantly (flattenScenario writes the whole
prompt in one go). It never waited for the terminal's column count to settle.

But a split kicks off a Lath tween that animates each pane's real width across
many frames (LathHost sets el.style.width per rAF), and TerminalPane's refit is
throttled with a 150ms trailing edge, so cols keeps reflowing well after the
content lands. settleTerminals resolved ~2 paint frames after content appeared,
so Chromatic sometimes snapshotted a pane still laid out at a transitional
width -- `user@dormouse:~$` clipped to `user@do`. Intermittent because the
two-frame cushion sometimes outlasted the throttle and sometimes didn't.

Gate on content AND settled geometry: each poll, force every live terminal to
fit its current resting container (bypassing the 150ms throttle so we read the
true resting cols), and require the grid to hold steady for STABLE_POLLS
consecutive polls before releasing the snapshot. Also await document.fonts.ready
first (bounded) since cell metrics are measured from the font. Public API
(settleTerminals / waitForCondition) is unchanged, so all consumer stories are
unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Terminal stories that split panes (Wall: Multi Pane, WithDoors) intermittently
snapshotted a pane with its last prompt line clipped — `user@dormouse:~$`
rendered as `user@do`.

Root cause: the split runs Lath's 440ms geometry tween, resizing panes through
many transient widths. xterm's DOM renderer can latch a frame where a pane is
still narrow and freeze its last line clipped to that width even after the
layout settles to full width. It's a rendering race on the animation, not a
content or column-count problem — the buffer is correct throughout.

Fix: disable Lath layout motion under Chromatic (cfg.layout.animate), the same
way preview.ts already freezes marching ants, the cursor blink, and the alert
ring for deterministic snapshots. Splits/restores/kills snap straight to their
final geometry, so no transient-width frame exists for the renderer to latch.
The animator already collapses to instant at durationMs 0 (the reduced-motion
path), so this reuses that code path. Snapshots capture the settled state, which
is identical with or without the tween — no visual change to any story.

This supersedes the earlier settle-terminals "settled grid geometry" gate, which
targeted column width and did not address the rendering race; settle-terminals
is reverted here to its original content-only form.

Validated with a headless-visible Playwright harness (mirrors Chromatic) over
the Wall: Multi Pane story: 30% truncation baseline → 0/150 with this change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>