test sweep: kernel commands + widgets coverage, ratchet floor to 93

Add tests for `keybindings.dart` (KeyEvent → Keybinding mapping,
parse-error edges, resolver entries view), `toolchain_paths.dart`
(the Flutter-free `ToolchainView.resolved` static view), and several
`widgets/src/` primitives: tooltip hover/overlay, palette filter +
submit, multitab controller `copyWith` + size getters, and additional
markdown branches (h3–h6 headings, tables, strikethrough, default
block fallback, record-link tap).

Unfreezes the pre-push coverage floor that was held at 90 on
2026-05-14 by mistake and ratchets to 93. Tidies eight test files
that had accumulated unused imports flagged by `unnecessary_import`.

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

pubspec.yaml

@@ -16,9 +16,9 @@ publish_to: none
 version: 2.0.0
 repository: https://github.com/postmeridiem/clide
 
-# Pre-push line-coverage floor. Held fixed at 90 — see D-66.
+# Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 90
+coverage_floor: 93
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads