dissolve app/ into repo root (D-056)

Single Flutter package at the repo root. All code, tests, assets,
and platform directories moved from app/ to root. Package renamed
from clide_app to clide — all imports rewritten. Merged pubspec
combines core (ffi) and app (flutter, yaml, xterm) dependencies.
Makefile simplified: no APP_PRESENT conditionals, no cd, no daemon
lifecycle. 317 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/dialog.dart

@@ -0,0 +1,108 @@
+import 'dart:async';
+
+import 'package:flutter/widgets.dart';
+
+typedef DialogBuilder<T> = Widget Function(
+  BuildContext context,
+  void Function([T? result]) dismiss,
+);
+
+/// Single-at-a-time modal router.
+///
+/// Extensions call [show] with a builder; the root widget (installed by
+/// [DialogHost]) listens and renders the current dialog over a dimmed
+/// backdrop. Only one dialog is active at a time — a second [show] call
+/// while one is open awaits until the first dismisses.
+class DialogRouter extends ChangeNotifier {
+  DialogBuilder<Object?>? _current;
+  Completer<Object?>? _completer;
+  final List<_Queued> _queue = [];
+
+  DialogBuilder<Object?>? get current => _current;
+  bool get isOpen => _current != null;
+
+  Future<T?> show<T extends Object>(DialogBuilder<T> builder) {
+    final completer = Completer<T?>();
+    final wrapped = _wrap<T>(builder);
+    if (_current == null) {
+      _current = wrapped;
+      _completer = Completer<Object?>();
+      // forward our generic completer to the typed one
+      _completer!.future.then((v) {
+        if (!completer.isCompleted) completer.complete(v as T?);
+      });
+      notifyListeners();
+    } else {
+      _queue.add(_Queued(wrapped, completer));
+    }
+    return completer.future;
+  }
+
+  void dismiss([Object? result]) {
+    if (_current == null) return;
+    final c = _completer;
+    _current = null;
+    _completer = null;
+    if (c != null && !c.isCompleted) c.complete(result);
+    if (_queue.isNotEmpty) {
+      final next = _queue.removeAt(0);
+      _current = next.builder;
+      _completer = Completer<Object?>();
+      _completer!.future.then((v) {
+        if (!next.completer.isCompleted) next.completer.complete(v);
+      });
+    }
+    notifyListeners();
+  }
+
+  DialogBuilder<Object?> _wrap<T>(DialogBuilder<T> builder) {
+    return (ctx, dismiss) => builder(ctx, ([T? v]) => dismiss(v));
+  }
+}
+
+class _Queued {
+  _Queued(this.builder, this.completer);
+  final DialogBuilder<Object?> builder;
+  // ignore: strict_raw_type
+  final Completer completer;
+}
+
+/// Hosts the current dialog from [DialogRouter]. Place high in the tree
+/// (inside the WidgetsApp) so dialogs overlay every other surface.
+class DialogHost extends StatelessWidget {
+  const DialogHost({
+    super.key,
+    required this.router,
+    required this.child,
+    this.backdropColor = const Color(0xC0000000),
+  });
+
+  final DialogRouter router;
+  final Widget child;
+  final Color backdropColor;
+
+  @override
+  Widget build(BuildContext context) {
+    return Stack(
+      fit: StackFit.expand,
+      children: [
+        child,
+        ListenableBuilder(
+          listenable: router,
+          builder: (ctx, _) {
+            final b = router.current;
+            if (b == null) return const SizedBox.shrink();
+            return Positioned.fill(
+              child: ColoredBox(
+                color: backdropColor,
+                child: Center(
+                  child: b(ctx, router.dismiss),
+                ),
+              ),
+            );
+          },
+        ),
+      ],
+    );
+  }
+}