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/pty/env_test.dart

@@ -36,29 +36,20 @@ void main() {
     test('on macOS, includes the well-known extras', () {
       if (!Platform.isMacOS) return;
       // Homebrew is the canonical one; at least one of these should appear.
-      expect(
-        expandedPath,
-        anyOf(contains('/opt/homebrew/bin'), contains('/usr/local/bin')),
-      );
+      expect(expandedPath, anyOf(contains('/opt/homebrew/bin'), contains('/usr/local/bin')));
     });
   });
 
   group('mergePtyEnv', () {
     test('clide defaults override the process env where they overlap', () {
-      final merged = mergePtyEnv(processEnv: {
-        'TERM': 'dumb',
-        'CUSTOM': 'preserved',
-      });
+      final merged = mergePtyEnv(processEnv: {'TERM': 'dumb', 'CUSTOM': 'preserved'});
       expect(merged['TERM'], 'xterm-256color'); // clide default wins
       expect(merged['COLORTERM'], 'truecolor');
       expect(merged['CUSTOM'], 'preserved'); // process env retained
     });
 
     test('overrides win over both process env and clide defaults', () {
-      final merged = mergePtyEnv(
-        processEnv: {'TERM': 'dumb'},
-        overrides: {'TERM': 'screen-256color'},
-      );
+      final merged = mergePtyEnv(processEnv: {'TERM': 'dumb'}, overrides: {'TERM': 'screen-256color'});
       expect(merged['TERM'], 'screen-256color');
     });