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/daemon/editor_commands_test.dart

@@ -138,9 +138,7 @@ void main() {
     await call('editor.open', {'path': 'a.md'});
     await call('editor.open', {'path': 'b.md'});
     final r = await call('editor.list');
-    final names = [
-      for (final b in (r.data['buffers'] as List).cast<Map>()) b['path'],
-    ];
+    final names = [for (final b in (r.data['buffers'] as List).cast<Map>()) b['path']];
     expect(names, containsAll(['a.md', 'b.md']));
   });
 
@@ -204,14 +202,14 @@ void main() {
   test('editor.set-selection clamps and applies', () async {
     await call('editor.open', {'path': 'doc.md'});
     final r = await call('editor.set-selection', {
-      'selection': {'start': 0, 'end': 3}
+      'selection': {'start': 0, 'end': 3},
     });
     expect(r.ok, isTrue);
   });
 
   test('editor.set-selection without an id or active buffer returns not-found', () async {
     final r = await call('editor.set-selection', {
-      'selection': {'start': 0, 'end': 1}
+      'selection': {'start': 0, 'end': 1},
     });
     expect(r.ok, isFalse);
     expect(r.error!.kind, 'not_found');
@@ -226,7 +224,7 @@ void main() {
     expect(read1.data['content'], 'replaced');
     final r2 = await call('editor.set-content', {
       'text': 'short',
-      'selection': {'start': 1, 'end': 99}
+      'selection': {'start': 1, 'end': 99},
     });
     expect(r2.ok, isTrue);
   });