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/daemon/image_commands_test.dart

@@ -56,6 +56,13 @@ void main() {
     expect(published.single.data, {'path': '/abs/docs/shot.png', 'caption': 'before', 'label': 'HUD v3', 'description': 'cramped status row'});
   });
 
+  test('a --stdin payload carries metadata like --file (T-315)', () async {
+    wire(found: {'docs/shot.png'});
+    final r = await show([], flags: {'stdin': '{"path":"docs/shot.png","label":"HUD"}'});
+    expect(r.ok, isTrue, reason: r.error?.message);
+    expect(published.single.data, {'path': '/abs/docs/shot.png', 'label': 'HUD'});
+  });
+
   test('a malformed --file payload is an honest userError, nothing published', () async {
     wire(files: {'meta.json': 'not json'});
     final r = await show([], flags: {'file': 'meta.json'});