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/builtin/deeplink/src/deep_link.dart

@@ -23,9 +23,9 @@ class DeepLinkAction {
 
   /// A human-readable description for the confirmation prompt.
   String get describe => switch (name) {
-        'open' => 'Open  $path${line != null ? '  (line $line)' : ''}',
-        _ => name,
-      };
+    'open' => 'Open  $path${line != null ? '  (line $line)' : ''}',
+    _ => name,
+  };
 }
 
 /// Parse [url] into a [DeepLinkAction], or null when it is malformed, not a

lib/builtin/deeplink/src/extension.dart

@@ -25,13 +25,8 @@ class DeepLinkExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-        CommandContribution(
-          id: 'deeplink.invoke',
-          command: 'deeplink.invoke',
-          title: 'Open a clide:// deep link',
-          run: _invoke,
-        ),
-      ];
+    CommandContribution(id: 'deeplink.invoke', command: 'deeplink.invoke', title: 'Open a clide:// deep link', run: _invoke),
+  ];
 
   @override
   Future<void> activate(ClideExtensionContext ctx) async => _ctx = ctx;