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/src/panes/pane.dart

@@ -16,23 +16,12 @@ enum PaneKind {
   String get wire => name;
 
   static PaneKind parse(String s) {
-    return PaneKind.values.firstWhere(
-      (v) => v.wire == s,
-      orElse: () => throw ArgumentError.value(s, 'kind', 'unknown pane kind'),
-    );
+    return PaneKind.values.firstWhere((v) => v.wire == s, orElse: () => throw ArgumentError.value(s, 'kind', 'unknown pane kind'));
   }
 }
 
 class Pane {
-  Pane({
-    required this.id,
-    required this.kind,
-    required this.pid,
-    required this.argv,
-    this.cwd,
-    this.title,
-    this.isClosed = false,
-  });
+  Pane({required this.id, required this.kind, required this.pid, required this.argv, this.cwd, this.title, this.isClosed = false});
 
   final String id;
   final PaneKind kind;
@@ -47,12 +36,12 @@ class Pane {
   bool isClosed;
 
   Map<String, Object?> toJson() => {
-        'id': id,
-        'kind': kind.wire,
-        'pid': pid,
-        'argv': argv,
-        if (cwd != null) 'cwd': cwd,
-        if (title != null) 'title': title,
-        'closed': isClosed,
-      };
+    'id': id,
+    'kind': kind.wire,
+    'pid': pid,
+    'argv': argv,
+    if (cwd != null) 'cwd': cwd,
+    if (title != null) 'title': title,
+    'closed': isClosed,
+  };
 }

lib/src/panes/registry.dart

@@ -52,36 +52,17 @@ class PaneRegistry {
       'COLORTERM': 'truecolor',
       'LANG': 'en_US.UTF-8',
       'LC_ALL': 'en_US.UTF-8',
-      if (env != null) ...env,
+      ...?env,
     };
 
-    final session = NativePty.start(
-      executable: executable,
-      arguments: arguments,
-      columns: cols,
-      rows: rows,
-      workingDirectory: cwd,
-      environment: fullEnv,
-    );
-    final pane = Pane(
-      id: id,
-      kind: kind,
-      pid: session.pid,
-      argv: argv,
-      cwd: cwd,
-      title: title,
-    );
+    final session = NativePty.start(executable: executable, arguments: arguments, columns: cols, rows: rows, workingDirectory: cwd, environment: fullEnv);
+    final pane = Pane(id: id, kind: kind, pid: session.pid, argv: argv, cwd: cwd, title: title);
     _panes[id] = pane;
     _sessions[id] = session;
 
     _emit('pane.spawned', id, pane.toJson());
 
-    _subs[id] = session.output.listen(
-      (bytes) => _emit('pane.output', id, {
-        'bytes_b64': base64Encode(bytes),
-      }),
-      onDone: () => _onExit(pane),
-    );
+    _subs[id] = session.output.listen((bytes) => _emit('pane.output', id, {'bytes_b64': base64Encode(bytes)}), onDone: () => _onExit(pane));
 
     return pane;
   }
@@ -136,11 +117,6 @@ class PaneRegistry {
   }
 
   void _emit(String kind, String id, Map<String, Object?> data) {
-    events.emit(IpcEvent(
-      subsystem: 'pane',
-      kind: kind,
-      timestamp: DateTime.now().toUtc(),
-      data: {'id': id, ...data},
-    ));
+    events.emit(IpcEvent(subsystem: 'pane', kind: kind, timestamp: DateTime.now().toUtc(), data: {'id': id, ...data}));
   }
 }

lib/src/panes/view_pane.dart

@@ -11,13 +11,7 @@
 library;
 
 class ViewPane {
-  const ViewPane({
-    required this.id,
-    required this.slot,
-    required this.title,
-    required this.active,
-    required this.visible,
-  });
+  const ViewPane({required this.id, required this.slot, required this.title, required this.active, required this.visible});
 
   /// Stable contribution id (e.g. `claude`, `files`, `editor`) — the same id
   /// `pane.focus` would target.
@@ -36,13 +30,5 @@ class ViewPane {
   /// Whether the tab's slot is currently visible (not collapsed/hidden).
   final bool visible;
 
-  Map<String, Object?> toJson() => {
-        'id': id,
-        'kind': 'view',
-        'slot': slot,
-        'title': title,
-        'active': active,
-        'visible': visible,
-        'source': 'ui',
-      };
+  Map<String, Object?> toJson() => {'id': id, 'kind': 'view', 'slot': slot, 'title': title, 'active': active, 'visible': visible, 'source': 'ui'};
 }