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>

test/kernel/src/keymap/shipped_presets_test.dart

@@ -16,6 +16,7 @@ library;
 import 'dart:io';
 
 import 'package:clide/kernel/src/keymap/intents.dart';
+import 'package:clide/kernel/src/keymap/key_chord.dart';
 import 'package:clide/kernel/src/keymap/keymap.dart';
 import 'package:flutter/widgets.dart' show NextFocusIntent, PreviousFocusIntent;
 import 'package:flutter_test/flutter_test.dart';
@@ -49,4 +50,16 @@ void main() {
     expect(layer.bindings.any((b) => b.intent is NextFocusIntent), isTrue);
     expect(layer.bindings.any((b) => b.intent is PreviousFocusIntent), isTrue);
   });
+
+  // Every shipped preset aliases the double-Shift "Search Everywhere"
+  // gesture to quick-open (T-341).
+  final doubleShift = [KeyChord.bareModifier(KeyModifier.shift), KeyChord.bareModifier(KeyModifier.shift)];
+  for (final file in presets) {
+    final name = file.uri.pathSegments.last;
+    test('$name resolves `shift shift` (double-tap) to quick-open', () {
+      final km = Keymap([KeymapLayer.fromYaml(file.readAsStringSync())]);
+      final m = km.match(doubleShift, const {});
+      expect(m.exact, isA<QuickOpenIntent>(), reason: '$name should bind double-Shift to quickOpen.open');
+    });
+  }
 }