test sweep: cover widgets/src zero-coverage primitives (T-91)

12 widget tests in test/widgets/zero_coverage_widgets_test.dart
covering six widgets that had 0% coverage:

- ClidePalette: closed → SizedBox.shrink; open → input + command
  list rendered; tap-to-invoke.
- ClideFilterBox: debounced onChanged (timer-flushed), clear button
  visibility + reset, onSubmitted forward.
- ColumnHat: left / center / right factories all render; center
  label joins project + branch with " > " separator; falls back to
  "clide" when neither set.
- ClideIconRail: one button per item, semantics labels, onSelect
  fires on tap.
- ClideSpine: label rotation + badge dot + onExpand tap; right-side
  variant.
- ClideResizeBorder: wraps child + renders 8 resize zones.

Coverage (totals lifted by these + the prior stale-lcov merge): jumps
from 83.92% to 86.58%. Floor bumped to 86.

Target reset to 90% per user direction: keep grinding sub-areas
until total clears 90%, then keep pushing as reasonable.

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: 83
+coverage_floor: 86
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads