chore: adopt Dart 3.9 toolchain — honest floor + tall-style reformat (T-353)

Raise the declared minimums in pubspec.yaml to what our deps already
require: Flutter >=3.35.0 / Dart >=3.9.0 (was 3.19.0 / 3.5.0). alchemist
0.12 needs Flutter 3.32; Dart 3.9 first ships in Flutter 3.35, so 3.35 is
the binding floor. Pin the exact build toolchain in .fvmrc (Flutter
3.44.1).

Moving to the Dart 3.9 language level switches `dart format` to the new
"tall" style and enables two new lints. This commit is the resulting
mechanical churn, isolated from any behaviour change:
  - whole-tree `dart format` reformat (tall style)
  - `dart fix` for unnecessary_underscores + use_null_aware_elements

No runtime behaviour change; `make test` green.

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

lib/builtin/claude/src/conversation_controller.dart

@@ -21,11 +21,8 @@ class ConversationController extends ChangeNotifier {
   /// over stream-json so the pane would otherwise start empty. [onDispose]
   /// is invoked from [dispose] — wire it to the reader's `dispose` so
   /// cancelling the view tears down the underlying tail.
-  ConversationController({
-    required Stream<ConversationItem> stream,
-    Iterable<ConversationItem>? seed,
-    Future<void> Function()? onDispose,
-  }) : _onDispose = onDispose {
+  ConversationController({required Stream<ConversationItem> stream, Iterable<ConversationItem>? seed, Future<void> Function()? onDispose})
+    : _onDispose = onDispose {
     if (seed != null) _items.addAll(seed);
     _sub = stream.listen(_onItem);
   }
@@ -34,13 +31,10 @@ class ConversationController extends ChangeNotifier {
   /// the [ConversationItem]s a [TranscriptPublisher] writes onto
   /// [publisher]/[channel]. Decouples the view from the reader so several
   /// panels can render the same conversation (team work, T-139/T-140).
-  factory ConversationController.fromBus({
-    required MessageBus messages,
-    String channel = ClaudeConversation.leadChannel,
-    Future<void> Function()? onDispose,
-  }) {
-    final stream =
-        messages.subscribe(publisher: ClaudeConversation.publisher, channel: channel).map((m) => m.data[ClaudeConversation.itemKey] as ConversationItem);
+  factory ConversationController.fromBus({required MessageBus messages, String channel = ClaudeConversation.leadChannel, Future<void> Function()? onDispose}) {
+    final stream = messages
+        .subscribe(publisher: ClaudeConversation.publisher, channel: channel)
+        .map((m) => m.data[ClaudeConversation.itemKey] as ConversationItem);
     return ConversationController(stream: stream, onDispose: onDispose);
   }