keymap: support bare-modifier double-tap chords; double-Shift → quick-open (T-341)

The chord matcher couldn't represent a bare or double-tapped modifier:
KeyChord.parse required a base key, so `shift shift` failed, and JetBrains
"Search Everywhere" (double-Shift) was unbindable.

Design decision: search-everywhere aliases clide's existing quick-open
finder (not a new overlay) — bound across all four presets per the user.

Changes:
- KeyChord: a bare modifier name (`shift`, `ctrl`, `cmd`, …) parses as a
  modifier-free chord on that modifier's logical key, so parseSequence(
  'shift shift') yields a two-chord double-tap. Adds KeyChord.bareModifier
  and modifierForLogicalKey.
- ModifierTapTracker: headless, clock-injected double-tap detector. A bare
  modifier never forms a single chord; an intervening key breaks the gesture.
- app.dart global handler feeds bare-modifier KeyDowns to the tracker and,
  on a double-tap, resolves the 2-chord sequence via the new
  KeymapService.resolveSequence. The existing single-chord path is untouched
  (zero behavioural risk to normal keys).
- Presets: default/vim/vscode/jetbrains add `shift shift` → quickOpen.open.
  jetbrains header updated (the gesture is now expressible).

Tests: bare-modifier parse/equality/round-trip; tracker window/reset/
different-modifier/consume; each shipped preset resolves double-Shift to
QuickOpenIntent.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/kernel/src/keymap/modifier_tap.dart

@@ -0,0 +1,44 @@
+/// Detects a double-tapped bare modifier (e.g. JetBrains "Search
+/// Everywhere" = double-Shift). (T-341)
+///
+/// Headless and clock-injected: the caller (the global key handler) passes
+/// the event time so it neither reads a clock nor consumes events. Feed it
+/// every [KeyDownEvent]: a bare modifier press via [tap], any other key via
+/// [reset] (an intervening key breaks the gesture, e.g. `Shift a Shift`).
+library;
+
+import 'key_chord.dart';
+
+class ModifierTapTracker {
+  ModifierTapTracker({this.window = const Duration(milliseconds: 350)});
+
+  /// Max gap between the two taps to count as a double-tap.
+  final Duration window;
+
+  KeyModifier? _last;
+  DateTime? _lastAt;
+
+  /// Record a bare-modifier press at [now]. Returns the modifier when this
+  /// press completes a double-tap of the *same* modifier within [window];
+  /// otherwise records it as the first tap and returns null.
+  KeyModifier? tap(KeyModifier m, DateTime now) {
+    final last = _last;
+    final lastAt = _lastAt;
+    if (last == m && lastAt != null) {
+      final gap = now.difference(lastAt);
+      if (gap >= Duration.zero && gap <= window) {
+        reset();
+        return m;
+      }
+    }
+    _last = m;
+    _lastAt = now;
+    return null;
+  }
+
+  /// Break the gesture — any non-modifier key press resets the tracker.
+  void reset() {
+    _last = null;
+    _lastAt = null;
+  }
+}