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/cli/argv_dispatch.dart

@@ -29,14 +29,12 @@ const String argvSentinelCmd = '_argv';
 ArgvParseResult unwrapArgvRequest(IpcRequest outer) {
   final raw = outer.args['argv'];
   if (raw is! List) {
-    return ArgvError(IpcResponse.err(
-      id: outer.id,
-      error: IpcError(
-        code: IpcExitCode.userError,
-        kind: IpcErrorKind.userError,
-        message: '_argv requires args.argv to be a JSON array',
+    return ArgvError(
+      IpcResponse.err(
+        id: outer.id,
+        error: IpcError(code: IpcExitCode.userError, kind: IpcErrorKind.userError, message: '_argv requires args.argv to be a JSON array'),
       ),
-    ));
+    );
   }
   return parseArgv(raw.cast<String>(), requestId: outer.id);
 }