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>

test/kernel/src/ipc/client_test.dart

@@ -113,15 +113,8 @@ void main() {
       await daemon.waitForClient();
       await Future<void>.delayed(const Duration(milliseconds: 50));
 
-      final eventFuture = bus.stream.firstWhere(
-        (e) => e.event is DaemonEvent,
-      );
-      final ev = IpcEvent(
-        subsystem: 'pty',
-        kind: 'output',
-        data: {'bytes': 'aGVsbG8='},
-        timestamp: DateTime.now(),
-      );
+      final eventFuture = bus.stream.firstWhere((e) => e.event is DaemonEvent);
+      final ev = IpcEvent(subsystem: 'pty', kind: 'output', data: {'bytes': 'aGVsbG8='}, timestamp: DateTime.now());
       daemon.send(ev.encode());
       final received = await eventFuture.timeout(const Duration(seconds: 2));
       final daemonEvent = received.event as DaemonEvent;