Two resize fixes for a steadier TUI under aggressive terminal resizing.
1. Drag-resize flicker (useMainApp): `cols` was synced to
`stdout.columns` synchronously on every 'resize' event. Each distinct
width remounts the visible transcript rows (they're keyed on cols so
yoga re-measures off live geometry), so a drag — which fires a burst of
resize events — turned into a per-tick remount storm that flickers and
stutters. Throttle the sync with a leading+trailing edge: the first
event reflows immediately (stays responsive), the rest collapse to at
most one reflow per RESIZE_COALESCE_MS (~30fps), and the trailing edge
always applies the final width so the settled layout is exact.
2. Resize-burst heal coverage (#18449): the existing ink-resize test only
exercised a single same-dimension event. Add two regressions that drive
a rapid resize *burst* (wobbling dims that settle back to the start, and
an isolated same-dimension event with no tree change) and assert the
renderer converges to a clean erased repaint — screen erased, then
content repainted after — rather than a partial diff over drifted cells.
This also relaxes the pre-existing single-event assertion, which
hard-coded the exact bytes `ESC[2J ESC[H`; the heal legitimately
interposes `ESC[3J` (erase scrollback) on some recovery paths, so all
three tests now assert the semantic invariant instead of a byte run.
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