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/builtin/default_layout/focus_exit_vim_test.dart

@@ -37,13 +37,16 @@ void main() {
       // Real layering: vim.yaml preset (low) under a contributions layer
       // carrying the extension's actual focusMode.exit escape binding.
       final preset = KeymapLayer.fromYaml(File('assets/keymaps/vim.yaml').readAsStringSync());
-      final contributions = KeymapLayer(name: 'contributions', bindings: [
-        KeymapBinding.chord(
-          KeyChord.parse(exitCmd.defaultBinding!),
-          intent: InvokeCommandIntent(exitCmd.command),
-          when: WhenExpr.tryParse(exitCmd.bindingWhen),
-        ),
-      ]);
+      final contributions = KeymapLayer(
+        name: 'contributions',
+        bindings: [
+          KeymapBinding.chord(
+            KeyChord.parse(exitCmd.defaultBinding!),
+            intent: InvokeCommandIntent(exitCmd.command),
+            when: WhenExpr.tryParse(exitCmd.bindingWhen),
+          ),
+        ],
+      );
       // Keymap flattens layers in reverse, so contributions outrank preset —
       // matching KeymapService._rebuildActive's ordering.
       km = Keymap([preset, contributions]);