test sweep: cover ui/render.dart (T-91)

10 widget tests driving RenderTerminal through a hosted TerminalView:
the reactive setters via updateRenderObject (theme, textStyle,
textScaler, padding, autoResize, cursorType, alwaysShowCursor) and
direct setter calls on the render box (padding, onEditableRect,
composingText) to cover the value-changed branches; getOffset for
non-origin cells; systemFontsDidChange; the terminal listener via
write(); the viewport-offset listener via scrollback overflow; and
the paint paths for composingText and controller-driven highlights.

Coverage: render.dart 179/249 -> 245/249 (98%). The 4 still-uncovered
lines are the _onScroll body — reachable only when the inner
ViewportOffset notifies, which doesn't happen with the current
ViewportOffset.zero() wiring (scroll lives in PointerScrollEvent →
PgUp/PgDown). Total 71.23% -> 72.02%; floor bumped to 72.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 71
+coverage_floor: 72
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads