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/pql_commands_test.dart

@@ -181,11 +181,7 @@ void main() {
   });
 
   test('pql.decisions.show forwards withRefs / withTickets', () async {
-    final r = await call('pql.decisions.show', {
-      'id': 'D-1',
-      'withRefs': true,
-      'withTickets': true,
-    });
+    final r = await call('pql.decisions.show', {'id': 'D-1', 'withRefs': true, 'withTickets': true});
     expect(r.ok, isTrue);
     expect(r.data['id'], 'D-1');
   });
@@ -198,12 +194,7 @@ void main() {
   });
 
   test('pql.tickets.list with filters narrows + shows status', () async {
-    final r = await call('pql.tickets.list', {
-      'status': 'done',
-      'team': 'whatever',
-      'assigned': 'no-one',
-      'decision': 'D-1',
-    });
+    final r = await call('pql.tickets.list', {'status': 'done', 'team': 'whatever', 'assigned': 'no-one', 'decision': 'D-1'});
     expect(r.ok, isTrue);
     expect(r.data['tickets'], isA<List>());
   });
@@ -212,11 +203,7 @@ void main() {
     final missing = await call('pql.tickets.show');
     expect(missing.ok, isFalse);
     expect(missing.error!.kind, 'user_error');
-    final ok = await call('pql.tickets.show', {
-      'id': 'T-1',
-      'withContext': true,
-      'withBlockers': true,
-    });
+    final ok = await call('pql.tickets.show', {'id': 'T-1', 'withContext': true, 'withBlockers': true});
     expect(ok.ok, isTrue);
   });