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/widgets/src/clide_pane_test.dart

@@ -17,14 +17,16 @@ void main() {
   tearDown(() => f.dispose());
 
   testWidgets('conveys its status while its contribution is focused, clears on blur', (tester) async {
-    await tester.pumpWidget(harness(
-      f,
-      const ClidePane(
-        contributionId: 'pane.x',
-        statusWidget: Text('S', textDirection: TextDirection.ltr),
-        child: SizedBox(),
+    await tester.pumpWidget(
+      harness(
+        f,
+        const ClidePane(
+          contributionId: 'pane.x',
+          statusWidget: Text('S', textDirection: TextDirection.ltr),
+          child: SizedBox(),
+        ),
       ),
-    ));
+    );
     final focus = f.services.focus;
     expect(focus.activeStatusWidget, isNull); // not focused yet
 
@@ -43,34 +45,38 @@ void main() {
     // mid-build ("markNeedsBuild during build"). A focus listener must be in
     // the tree to reproduce it.
     f.services.focus.setActive(slot: Slots.workspace, contributionId: 'pane.x');
-    await tester.pumpWidget(harness(
-      f,
-      const Column(
-        children: [
-          PaneContextStatusItem(),
-          ClidePane(
-            contributionId: 'pane.x',
-            statusWidget: Text('S', textDirection: TextDirection.ltr),
-            child: SizedBox(),
-          ),
-        ],
+    await tester.pumpWidget(
+      harness(
+        f,
+        const Column(
+          children: [
+            PaneContextStatusItem(),
+            ClidePane(
+              contributionId: 'pane.x',
+              statusWidget: Text('S', textDirection: TextDirection.ltr),
+              child: SizedBox(),
+            ),
+          ],
+        ),
       ),
-    ));
+    );
     await tester.pump(); // run the deferred post-frame convey
     expect(tester.takeException(), isNull);
     expect(f.services.focus.activeStatusWidget, isA<Text>());
   });
 
   testWidgets('an inactive pane never conveys', (tester) async {
-    await tester.pumpWidget(harness(
-      f,
-      const ClidePane(
-        contributionId: 'pane.x',
-        active: false,
-        statusWidget: Text('S', textDirection: TextDirection.ltr),
-        child: SizedBox(),
+    await tester.pumpWidget(
+      harness(
+        f,
+        const ClidePane(
+          contributionId: 'pane.x',
+          active: false,
+          statusWidget: Text('S', textDirection: TextDirection.ltr),
+          child: SizedBox(),
+        ),
       ),
-    ));
+    );
     f.services.focus.setActive(slot: Slots.workspace, contributionId: 'pane.x');
     await tester.pump();
     expect(f.services.focus.activeStatusWidget, isNull);
@@ -80,19 +86,21 @@ void main() {
     f.services.focus.setActive(slot: Slots.workspace, contributionId: 'pane.x');
     var label = 'A';
     late StateSetter setLabel;
-    await tester.pumpWidget(harness(
-      f,
-      StatefulBuilder(
-        builder: (ctx, setState) {
-          setLabel = setState;
-          return ClidePane(
-            contributionId: 'pane.x',
-            statusWidget: Text(label, textDirection: TextDirection.ltr),
-            child: const SizedBox(),
-          );
-        },
+    await tester.pumpWidget(
+      harness(
+        f,
+        StatefulBuilder(
+          builder: (ctx, setState) {
+            setLabel = setState;
+            return ClidePane(
+              contributionId: 'pane.x',
+              statusWidget: Text(label, textDirection: TextDirection.ltr),
+              child: const SizedBox(),
+            );
+          },
+        ),
       ),
-    ));
+    );
     await tester.pump();
     expect((f.services.focus.activeStatusWidget! as Text).data, 'A');