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/builtin/pql/src/pql_controller.dart

@@ -71,10 +71,7 @@ class PqlController extends ChangeNotifier {
     _error = null;
     notifyListeners();
 
-    final r = await ipc.request('pql.search', args: {
-      'terms': terms,
-      'limit': 50,
-    });
+    final r = await ipc.request('pql.search', args: {'terms': terms, 'limit': 50});
 
     _loading = false;
     if (!r.ok) {
@@ -91,10 +88,7 @@ class PqlController extends ChangeNotifier {
     _loading = true;
     notifyListeners();
 
-    final r = await ipc.request('pql.files', args: {
-      'glob': glob ?? '**/*.md',
-      'limit': 200,
-    });
+    final r = await ipc.request('pql.files', args: {'glob': glob ?? '**/*.md', 'limit': 200});
 
     _loading = false;
     if (!r.ok) {
@@ -113,10 +107,7 @@ class PqlController extends ChangeNotifier {
     _error = null;
     notifyListeners();
 
-    final r = await ipc.request('pql.query', args: {
-      'query': dsl,
-      'limit': 200,
-    });
+    final r = await ipc.request('pql.query', args: {'query': dsl, 'limit': 200});
 
     _loading = false;
     if (!r.ok) {