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/log_test.dart

@@ -5,10 +5,7 @@ void main() {
   group('Logger', () {
     test('respects minLevel — lower-level messages drop silently', () {
       final out = <LogRecord>[];
-      final log = Logger(
-        minLevel: LogLevel.warn,
-        sinks: [out.add],
-      );
+      final log = Logger(minLevel: LogLevel.warn, sinks: [out.add]);
       log.debug('s', 'dropped');
       log.info('s', 'dropped');
       log.warn('s', 'kept');
@@ -38,10 +35,7 @@ void main() {
 
     test('broken sink does not kill the logger', () {
       final good = <LogRecord>[];
-      final log = Logger(minLevel: LogLevel.info, sinks: [
-        (_) => throw StateError('bad sink'),
-        good.add,
-      ]);
+      final log = Logger(minLevel: LogLevel.info, sinks: [(_) => throw StateError('bad sink'), good.add]);
       log.info('s', 'still delivered');
       expect(good, hasLength(1));
     });