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/search/grep_engine_test.dart

@@ -74,10 +74,7 @@ void main() {
   });
 
   test('invalid regex throws FormatException', () async {
-    expect(
-      () => run(const SearchQuery(pattern: '(unclosed', regex: true)),
-      throwsA(isA<FormatException>()),
-    );
+    expect(() => run(const SearchQuery(pattern: '(unclosed', regex: true)), throwsA(isA<FormatException>()));
   });
 
   test('include glob restricts to matching files', () async {

test/search/replace_engine_test.dart

@@ -125,7 +125,11 @@ void main() {
     });
 
     test('ReplacementEdit + FileReplacement round-trip JSON', () {
-      const fr = FileReplacement(path: 'a.dart', count: 1, edits: [ReplacementEdit(line: 2, before: 'a', after: 'b')]);
+      const fr = FileReplacement(
+        path: 'a.dart',
+        count: 1,
+        edits: [ReplacementEdit(line: 2, before: 'a', after: 'b')],
+      );
       final back = FileReplacement.fromJson(fr.toJson());
       expect(back.path, 'a.dart');
       expect(back.count, 1);