feat(cli): --stdin piped JSON payloads for structured commands (T-315)

The C client slurps stdin when it sees --stdin, strips the flag, and ships
the payload alongside the argv; the Dart unwrap folds it into the request
as a `stdin` arg (undeclared keys pass the schema untouched). icon.show and
image.show now read that payload as the peer of --file (stdin wins). Bounded
slurp + the envelope's existing size guard keep a huge payload from
corrupting the wire. C client builds clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/cli/argv_dispatch_test.dart

@@ -6,6 +6,7 @@
 library;
 
 import 'package:clide/src/cli/argv_dispatch.dart';
+import 'package:clide/src/cli/argv_to_request.dart' show ArgvParsed;
 import 'package:clide/src/daemon/dispatcher.dart';
 import 'package:clide/src/ipc/envelope.dart';
 import 'package:clide/src/ipc/schema_v1.dart';
@@ -34,6 +35,23 @@ void main() {
     expect(res.data['pong'], isTrue);
   });
 
+  test('a stdin payload folds into the inner request as a flag (T-315)', () {
+    final result = unwrapArgvRequest(
+      IpcRequest(
+        id: 'x',
+        cmd: argvSentinelCmd,
+        args: {
+          'argv': ['icon', 'show'],
+          'stdin': '[{"icon":"gear"}]',
+        },
+      ),
+    );
+    expect(result, isA<ArgvParsed>());
+    final req = (result as ArgvParsed).request;
+    expect(req.cmd, 'icon.show');
+    expect((req.args['flags'] as Map)['stdin'], '[{"icon":"gear"}]');
+  });
+
   test('args.argv missing → userError', () async {
     final res = await d.dispatch(IpcRequest(id: 'y', cmd: argvSentinelCmd, args: const {}));
     expect(res.ok, isFalse);