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/loader.dart

@@ -70,10 +70,7 @@ class ThemeLoader {
     final surface = doc['surface'];
     final extension = doc['extension'];
 
-    final mergedSurface = <String, String>{
-      ...syntaxSurface,
-      if (surface is Map) ..._parseRefMap(surface),
-    };
+    final mergedSurface = <String, String>{...syntaxSurface, if (surface is Map) ..._parseRefMap(surface)};
 
     return ThemeDefinition(
       name: name,