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/src/files/watcher.dart

@@ -38,11 +38,7 @@ enum FileChangeKind {
 }
 
 class FileChange {
-  const FileChange({
-    required this.kind,
-    required this.path,
-    required this.isDirectory,
-  });
+  const FileChange({required this.kind, required this.path, required this.isDirectory});
 
   final FileChangeKind kind;
 
@@ -50,11 +46,7 @@ class FileChange {
   final String path;
   final bool isDirectory;
 
-  Map<String, Object?> toJson() => {
-        'kind': kind.wire,
-        'path': path,
-        'isDirectory': isDirectory,
-      };
+  Map<String, Object?> toJson() => {'kind': kind.wire, 'path': path, 'isDirectory': isDirectory};
 }
 
 class FileWatcher {
@@ -70,10 +62,7 @@ class FileWatcher {
 
   Future<void> start() async {
     if (_sub != null) return;
-    _sub = root.watch(recursive: true).listen(
-          _onEvent,
-          onError: (Object e, StackTrace _) => _controller.addError(e),
-        );
+    _sub = root.watch(recursive: true).listen(_onEvent, onError: (Object e, StackTrace _) => _controller.addError(e));
   }
 
   Future<void> stop() async {
@@ -87,11 +76,7 @@ class FileWatcher {
     if (rel == null) return;
     final isDir = e.isDirectory;
     if (_ignored(rel, isDir)) return;
-    _controller.add(FileChange(
-      kind: FileChangeKind.fromEvent(e),
-      path: rel,
-      isDirectory: isDir,
-    ));
+    _controller.add(FileChange(kind: FileChangeKind.fromEvent(e), path: rel, isDirectory: isDir));
   }
 
   /// True if `rel` is ignored, or sits inside an ignored directory.