test sweep: cover i18n catalog loaders + theme_picker _pick (T-91)

Two test files / batches:

- test/kernel/src/i18n/catalog_loader_test.dart (11 tests): all three
  CatalogLoader implementations end-to-end. AssetCatalogLoader against
  an in-memory AssetBundle covers happy path, FlutterError catch
  (missing asset), FormatException catch (malformed JSON), blank
  asset, non-object JSON. FileCatalogLoader against a tempdir covers
  happy path, missing file, malformed, blank. InMemoryCatalogLoader
  covers lookup + namespace-miss.

- Extends test/builtin/theme_picker/widget_test.dart with two tests
  for the _pick command callback: defensive not-activated branch
  (drive the contribution's run() before activate runs, _ctx is still
  null) and the dialog-resolution happy path (register + activate,
  invoke theme.pick, manually drive dialog.dismiss('forest'), assert
  IpcResponse data['selected']).

Coverage: kernel/src/i18n/catalog_loader.dart 14/26 -> 26/26;
builtin/theme_picker/src/extension.dart 9/18 -> 15/18 (remaining 3
lines are the dialog builder body — needs a DialogHost in the test
harness, out of scope).

Total coverage 74.08% -> 74.55%; floor bumped to 74.

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

test/kernel/src/i18n/catalog_loader_test.dart

@@ -0,0 +1,124 @@
+/// Tests for the three CatalogLoader implementations in
+/// `lib/kernel/src/i18n/catalog_loader.dart`. AssetCatalogLoader is
+/// exercised via an in-memory AssetBundle; FileCatalogLoader via a
+/// tempdir; InMemoryCatalogLoader inline.
+library;
+
+import 'dart:convert';
+import 'dart:io';
+import 'dart:typed_data';
+import 'dart:ui';
+
+import 'package:clide/kernel/src/i18n/catalog_loader.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+import 'package:test/test.dart';
+
+class _MapAssetBundle extends CachingAssetBundle {
+  _MapAssetBundle(this._files);
+  final Map<String, String> _files;
+
+  @override
+  Future<ByteData> load(String key) async {
+    final v = _files[key];
+    if (v == null) throw FlutterError('asset not found: $key');
+    final bytes = utf8.encode(v);
+    return ByteData.view(Uint8List.fromList(bytes).buffer);
+  }
+}
+
+void main() {
+  group('AssetCatalogLoader', () {
+    test('returns parsed JSON for a present asset', () async {
+      final bundle = _MapAssetBundle({
+        'lib/kernel/src/i18n/catalog/welcome_en_us.json': '{"title":{"translation":"Hi"}}',
+      });
+      final loader = AssetCatalogLoader(bundle: bundle);
+      final r = await loader.load('welcome', const Locale('en', 'US'));
+      expect(r['title'], isA<Map>());
+    });
+
+    test('returns an empty map when the asset is missing (FlutterError catch)', () async {
+      final loader = AssetCatalogLoader(bundle: _MapAssetBundle(const {}));
+      final r = await loader.load('nope', const Locale('en', 'US'));
+      expect(r, isEmpty);
+    });
+
+    test('returns an empty map on malformed JSON (FormatException catch)', () async {
+      final bundle = _MapAssetBundle({
+        'lib/kernel/src/i18n/catalog/welcome_en_us.json': 'not json at all',
+      });
+      final loader = AssetCatalogLoader(bundle: bundle);
+      expect(await loader.load('welcome', const Locale('en', 'US')), isEmpty);
+    });
+
+    test('returns an empty map when the asset is blank', () async {
+      final bundle = _MapAssetBundle({
+        'lib/kernel/src/i18n/catalog/welcome_en_us.json': '   \n',
+      });
+      final loader = AssetCatalogLoader(bundle: bundle);
+      expect(await loader.load('welcome', const Locale('en', 'US')), isEmpty);
+    });
+
+    test('returns an empty map when JSON parses to a non-object', () async {
+      final bundle = _MapAssetBundle({
+        'lib/kernel/src/i18n/catalog/welcome_en_us.json': '[1, 2, 3]',
+      });
+      final loader = AssetCatalogLoader(bundle: bundle);
+      expect(await loader.load('welcome', const Locale('en', 'US')), isEmpty);
+    });
+  });
+
+  group('FileCatalogLoader', () {
+    late Directory tmp;
+
+    setUp(() async {
+      tmp = await Directory.systemTemp.createTemp('clide-catalog-');
+    });
+
+    tearDown(() async {
+      if (tmp.existsSync()) await tmp.delete(recursive: true);
+    });
+
+    test('reads and parses a present file', () async {
+      await File('${tmp.path}/welcome_en_us.json').writeAsString('{"k":{"translation":"v"}}');
+      final loader = FileCatalogLoader(rootDir: tmp);
+      final r = await loader.load('welcome', const Locale('en', 'US'));
+      expect(r['k'], isA<Map>());
+    });
+
+    test('returns an empty map when the file is missing', () async {
+      final loader = FileCatalogLoader(rootDir: tmp);
+      expect(await loader.load('nope', const Locale('en', 'US')), isEmpty);
+    });
+
+    test('returns an empty map on malformed JSON (FormatException catch)', () async {
+      await File('${tmp.path}/welcome_en_us.json').writeAsString('garbage');
+      final loader = FileCatalogLoader(rootDir: tmp);
+      expect(await loader.load('welcome', const Locale('en', 'US')), isEmpty);
+    });
+
+    test('returns an empty map when the file is blank', () async {
+      await File('${tmp.path}/welcome_en_us.json').writeAsString('   ');
+      final loader = FileCatalogLoader(rootDir: tmp);
+      expect(await loader.load('welcome', const Locale('en', 'US')), isEmpty);
+    });
+  });
+
+  group('InMemoryCatalogLoader', () {
+    test('returns the catalog when (namespace, locale) matches', () async {
+      final loader = InMemoryCatalogLoader({
+        'welcome': {
+          const Locale('en', 'US'): const {'k': 'v'},
+        },
+      });
+      final r = await loader.load('welcome', const Locale('en', 'US'));
+      expect(r['k'], 'v');
+    });
+
+    test('returns an empty map when namespace is missing', () async {
+      final loader = InMemoryCatalogLoader(const {});
+      expect(await loader.load('nope', const Locale('en', 'US')), isEmpty);
+    });
+  });
+}