test sweep: cover default_layout commands (T-91)

Eight new tests in test/builtin/default_layout/widget_test.dart
covering the DefaultLayoutExtension command handlers the existing
four-test suite didn't reach:

- All commands return the _notActivated() error pre-activate (sweeps
  every command's defensive null-context branch in one loop).
- palette.toggle flips the palette's open state.
- sidebar.collapse + context.collapse toggle their slots.
- panel.focus.left / .middle / .right — including the auto-expand
  branches when a focused side is collapsed.
- panel.focusMode toggles focus mode on the active slot.
- panel.focusMode.exit unwinds in order: focus mode → editor → palette,
  with the empty-data no-op fallback.
- editor.open + editor.close including the close-already-closed
  no-op.
- sidebar.section.N: auto-expand sidebar + no-op when no tabs are
  contributed.

Coverage: builtin/default_layout/src/extension.dart 60/167 -> 139/167
(83%). Remaining 28 lines are the _restoreLayout / _persistLayout
persistence paths and the section-activation happy path, which need
a populated tab list + a projectDir on settings to exercise — out
of scope for this batch.

Total coverage 79.80% -> 80.81%; floor bumped to 80.

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: 79
+coverage_floor: 80
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads