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/kernel/src/theme/semantic.dart

@@ -24,17 +24,5 @@ abstract class SemanticKeys {
   static const error = 'error';
   static const info = 'info';
 
-  static const all = <String>[
-    mainchrome,
-    calltoaction,
-    focus,
-    background,
-    surface,
-    text,
-    textMuted,
-    success,
-    warning,
-    error,
-    info,
-  ];
+  static const all = <String>[mainchrome, calltoaction, focus, background, surface, text, textMuted, success, warning, error, info];
 }