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/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;