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/commands/keybindings.dart

@@ -5,9 +5,7 @@ import 'package:flutter/services.dart';
 /// (modifiers sorted, lowercased) so equality works for lookup keys.
 @immutable
 class Keybinding {
-  Keybinding({required Set<String> modifiers, required String key})
-      : modifiers = _canonModifiers(modifiers),
-        key = key.toLowerCase();
+  Keybinding({required Set<String> modifiers, required String key}) : modifiers = _canonModifiers(modifiers), key = key.toLowerCase();
 
   final List<String> modifiers;
   final String key;

lib/kernel/src/commands/registry.dart

@@ -17,19 +17,12 @@ class CommandRegistry extends ChangeNotifier {
   Iterable<CommandContribution> get all => _byCommand.values;
   CommandContribution? get(String command) => _byCommand[command];
 
-  Future<IpcResponse> execute(
-    String command, {
-    List<String> args = const [],
-  }) async {
+  Future<IpcResponse> execute(String command, {List<String> args = const []}) async {
     final c = _byCommand[command];
     if (c == null) {
       return IpcResponse.err(
         id: '',
-        error: IpcError(
-          code: IpcExitCode.notFound,
-          kind: IpcErrorKind.notFound,
-          message: 'no such command: $command',
-        ),
+        error: IpcError(code: IpcExitCode.notFound, kind: IpcErrorKind.notFound, message: 'no such command: $command'),
       );
     }
     return c.run(args);