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/kernel/src/toast.dart

@@ -13,27 +13,17 @@ enum ToastSeverity { success, warning, error, info }
 const String toastChannel = 'toast';
 
 ToastSeverity _severityFromName(Object? name) => switch (name) {
-      'success' => ToastSeverity.success,
-      'warning' => ToastSeverity.warning,
-      'error' => ToastSeverity.error,
-      _ => ToastSeverity.info,
-    };
+  'success' => ToastSeverity.success,
+  'warning' => ToastSeverity.warning,
+  'error' => ToastSeverity.error,
+  _ => ToastSeverity.info,
+};
 
 /// Raise a toast by publishing to the MessageBus — the decoupled path:
 /// emitters depend only on the bus, never on the [ToastService]. [publisher]
 /// is the emitter id (e.g. `builtin.git`), kept for provenance/filtering.
-void publishToast(
-  MessageBus messages,
-  String publisher,
-  String message, {
-  ToastSeverity severity = ToastSeverity.info,
-  Duration? duration,
-}) {
-  messages.publish(publisher, toastChannel, {
-    'message': message,
-    'severity': severity.name,
-    if (duration != null) 'durationMs': duration.inMilliseconds,
-  });
+void publishToast(MessageBus messages, String publisher, String message, {ToastSeverity severity = ToastSeverity.info, Duration? duration}) {
+  messages.publish(publisher, toastChannel, {'message': message, 'severity': severity.name, if (duration != null) 'durationMs': duration.inMilliseconds});
 }
 
 /// One live toast. Immutable; the [ToastService] owns the list.