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/terminal/coverage_trivials_test.dart

@@ -49,11 +49,7 @@ void main() {
     // CJK ideograph — terminal stores it as a width-2 cell, so paintLine
     // advances `i` twice and the wide-char branch fires.
     t.write('漢字');
-    final p = TerminalPainter(
-      theme: TerminalThemes.defaultTheme,
-      textStyle: const TerminalStyle(),
-      textScaler: const TextScaler.linear(1.0),
-    );
+    final p = TerminalPainter(theme: TerminalThemes.defaultTheme, textStyle: const TerminalStyle(), textScaler: const TextScaler.linear(1.0));
     p.paintLine(Canvas(PictureRecorder()), Offset.zero, t.buffer.lines[0]);
   });
 }