test sweep: cover ui/ widget-level helpers (T-91)

Adds test/terminal/ui/ui_widget_test.dart — 14 widget tests
covering the lib/src/terminal/src/ui/ helpers that need a Flutter
widget tree (CustomKeyboardListener, KeyboardVisibilty,
InfiniteScrollView, TerminalScrollGestureHandler).

Files reaching ~100% (the 4 missing lines in infinite_scroll_view
sit in the render-object's `position` setter, only triggered when
Scrollable feeds a different ViewportOffset between rebuilds —
not reachable through normal widget plumbing without internal
access):

- scroll_handler.dart: 0 / unmeasured → 39/39 (100%) — main-buffer
  passthrough vs. alt-buffer interception, mouse-mode forwarding
  vs. simulateScroll fallback, simulateScroll=false drop, alt-flip
  + didUpdateWidget rebinding, onPointerDown tracking the cursor
  for the next scroll event.
- infinite_scroll_view.dart: 0 / unmeasured → 36/40 (90%) — onScroll
  fires on viewport position change, callback identity update via
  updateRenderObject.
- keyboard_listener.dart: 7/12 → 12/12 (100%) — character-key
  fallthrough into onInsert when onKeyEvent returns ignored,
  short-circuit to onKeyEvent's "handled" return, no-op on a key
  with no character.
- keyboard_visibility.dart: 18/19 → 19/19 (100%) — show + hide
  callbacks paired against view-insets transitions; no fire on
  same-inset metrics events.

Coverage delta:
- Total project: 68.80% → 69.12%; coverage_floor bumped 68 → 69.

Tests use a `_host()` helper that wraps the widget under test in
Directionality + MediaQuery + Center + a sized SizedBox. The
TerminalScrollGestureHandler tests use ColoredBox as the child
because Listener.onPointerSignal needs a hit-testable render
object below it, and SizedBox.expand alone doesn't paint anything.

Co-Authored-By: Claude <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: 68
+coverage_floor: 69
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads