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/cli/argv_dispatch_test.dart

@@ -20,9 +20,15 @@ void main() {
   });
 
   test('valid argv → parsed and re-dispatched (round-trip via ping)', () async {
-    final res = await d.dispatch(IpcRequest(id: 'x', cmd: argvSentinelCmd, args: {
-      'argv': ['ping'],
-    }));
+    final res = await d.dispatch(
+      IpcRequest(
+        id: 'x',
+        cmd: argvSentinelCmd,
+        args: {
+          'argv': ['ping'],
+        },
+      ),
+    );
     expect(res.ok, isTrue);
     expect(res.id, 'x');
     expect(res.data['pong'], isTrue);
@@ -36,26 +42,36 @@ void main() {
   });
 
   test('args.argv is not a list → userError', () async {
-    final res = await d.dispatch(IpcRequest(id: 'z', cmd: argvSentinelCmd, args: {
-      'argv': 'not a list',
-    }));
+    final res = await d.dispatch(IpcRequest(id: 'z', cmd: argvSentinelCmd, args: {'argv': 'not a list'}));
     expect(res.ok, isFalse);
     expect(res.error?.kind, IpcErrorKind.userError);
   });
 
   test('argv that fails parseArgv → that error flows back unmodified', () async {
-    final res = await d.dispatch(IpcRequest(id: 'p', cmd: argvSentinelCmd, args: {
-      'argv': const <String>[], // empty argv triggers parseArgv usage error
-    }));
+    final res = await d.dispatch(
+      IpcRequest(
+        id: 'p',
+        cmd: argvSentinelCmd,
+        args: {
+          'argv': const <String>[], // empty argv triggers parseArgv usage error
+        },
+      ),
+    );
     expect(res.ok, isFalse);
     expect(res.error?.kind, IpcErrorKind.userError);
     expect(res.error?.message, contains('usage'));
   });
 
   test('outer request id is preserved on the response', () async {
-    final res = await d.dispatch(IpcRequest(id: 'unique-id-123', cmd: argvSentinelCmd, args: {
-      'argv': ['ping'],
-    }));
+    final res = await d.dispatch(
+      IpcRequest(
+        id: 'unique-id-123',
+        cmd: argvSentinelCmd,
+        args: {
+          'argv': ['ping'],
+        },
+      ),
+    );
     expect(res.id, 'unique-id-123');
   });
 }

test/cli/clide_cli_e2e_test.dart

@@ -36,14 +36,7 @@ void main() {
     if (!hasCC) return;
     final src = '$repoRoot/native/clide-cli/clide.c';
     final out = '${Directory.systemTemp.createTempSync('clide-cli-test-').path}/clide';
-    final build = await Process.run('cc', [
-      '-std=c99',
-      '-O2',
-      '-Wall',
-      src,
-      '-o',
-      out,
-    ]);
+    final build = await Process.run('cc', ['-std=c99', '-O2', '-Wall', src, '-o', out]);
     expect(build.exitCode, 0, reason: 'cc failed: ${build.stderr}');
     binaryPath = out;
 

test/cli/clide_version_test.dart

@@ -26,10 +26,6 @@ String _pubspecVersion() {
 
 void main() {
   test('clideVersion matches pubspec.yaml version (T-213)', () {
-    expect(
-      clideVersion,
-      _pubspecVersion(),
-      reason: 'build_info.g.dart drifted from pubspec.yaml — run `make gen-build-info`',
-    );
+    expect(clideVersion, _pubspecVersion(), reason: 'build_info.g.dart drifted from pubspec.yaml — run `make gen-build-info`');
   });
 }