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/ipc/mcp_server.dart

@@ -63,14 +63,7 @@ class _McpSession {
 /// HTTP + SSE MCP server. Lifecycle mirrors [IpcServer]: `start()`
 /// binds + writes the discovery file; `stop()` unbinds + removes it.
 class McpServer {
-  McpServer({
-    required this.workspaceRoot,
-    required this.log,
-    this.dispatcher,
-    this.discoveryDirOverride,
-    this.bindHost = '127.0.0.1',
-    this.bindPort = 0,
-  });
+  McpServer({required this.workspaceRoot, required this.log, this.dispatcher, this.discoveryDirOverride, this.bindHost = '127.0.0.1', this.bindPort = 0});
 
   /// Workspace root reported in the discovery file. Helps Claude
   /// Code show "which clide is this" when multiple are running.
@@ -109,9 +102,12 @@ class McpServer {
     _http = server;
     _port = server.port;
     _lockFile = await _writeDiscoveryFile();
-    server.listen(_route, onError: (Object e, StackTrace st) {
-      log.warn('mcp', 'http error: $e');
-    });
+    server.listen(
+      _route,
+      onError: (Object e, StackTrace st) {
+        log.warn('mcp', 'http error: $e');
+      },
+    );
     log.info('mcp', 'MCP/SSE listening at http://$bindHost:${server.port} (workspace: $workspaceRoot)');
   }
 
@@ -331,12 +327,7 @@ class McpServer {
       dirHandle.createSync(recursive: true);
     }
     final path = '$dir/$pid.lock';
-    final body = jsonEncode({
-      'pid': pid,
-      'workspace': workspaceRoot,
-      'transport': 'sse',
-      'url': 'http://$bindHost:$_port/sse',
-    });
+    final body = jsonEncode({'pid': pid, 'workspace': workspaceRoot, 'transport': 'sse', 'url': 'http://$bindHost:$_port/sse'});
     File(path).writeAsStringSync(body);
     return path;
   }