add quick-open fuzzy file finder (Ctrl/Cmd+P)

A file picker overlay over the whole workspace, distinct from the
command palette. QuickOpenController holds the file list + a
subsequence fuzzy filter; the overlay loads the list via files.walk on
open, shows RecentFilesService entries on an empty query, and opens the
selection through a shared openWorkspaceFile helper (.md → markdown
reader bus, else editor.open) that the files panel now also routes
through, so recents stay in sync from every open site.

Bound to ctrl+p / meta+p with `when: !palette.open` so it never
collides with the palette's ctrl+p navigation; in-overlay arrows/enter/
escape reuse the palette's keymap-driven model via quickOpen.* intents.

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

test/kernel/recent_files_test.dart

@@ -0,0 +1,56 @@
+/// Unit tests for [RecentFilesService] — the session-scoped recent
+/// files list backing quick-open's empty-query state (T-51).
+library;
+
+import 'package:clide/kernel/src/recent_files.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('push records most-recent-first', () {
+    final r = RecentFilesService();
+    r.push('a.dart');
+    r.push('b.dart');
+    expect(r.paths, ['b.dart', 'a.dart']);
+  });
+
+  test('push de-duplicates and moves an existing entry to the front', () {
+    final r = RecentFilesService();
+    r.push('a.dart');
+    r.push('b.dart');
+    r.push('a.dart');
+    expect(r.paths, ['a.dart', 'b.dart']);
+  });
+
+  test('trims to the cap, dropping the oldest', () {
+    final r = RecentFilesService(cap: 2);
+    r.push('a');
+    r.push('b');
+    r.push('c');
+    expect(r.paths, ['c', 'b']);
+  });
+
+  test('empty path is ignored', () {
+    final r = RecentFilesService();
+    r.push('');
+    expect(r.paths, isEmpty);
+  });
+
+  test('clear empties the list and notifies once', () {
+    final r = RecentFilesService();
+    var notifications = 0;
+    r.addListener(() => notifications++);
+    r.push('a');
+    r.clear();
+    expect(r.paths, isEmpty);
+    expect(notifications, 2);
+    // A second clear on an empty list is a no-op (no extra notify).
+    r.clear();
+    expect(notifications, 2);
+  });
+
+  test('paths is an unmodifiable snapshot', () {
+    final r = RecentFilesService();
+    r.push('a');
+    expect(() => r.paths.add('b'), throwsUnsupportedError);
+  });
+}