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>

lib/src/daemon/icon_commands.dart

@@ -62,11 +62,14 @@ Future<IpcResponse> _show(IpcRequest req, MessagePublisher? Function() publisher
   }
 
   final entries = <Map<String, Object?>>[];
+  // The entry payload comes from a piped --stdin (T-315) or a --file; --stdin
+  // wins. Either is a JSON array of {icon,label,description,color}.
+  final stdin = _str(req.args['stdin']);
   final file = _str(req.args['file']);
-
-  if (file != null) {
-    final raw = readFile == null ? null : await readFile(file);
-    if (raw == null) {
+  String? payload = stdin;
+  if (payload == null && file != null) {
+    payload = readFile == null ? null : await readFile(file);
+    if (payload == null) {
       return IpcResponse.err(
         id: req.id,
         error: IpcError(
@@ -77,11 +80,14 @@ Future<IpcResponse> _show(IpcRequest req, MessagePublisher? Function() publisher
         ),
       );
     }
+  }
+
+  if (payload != null) {
     Object? decoded;
     try {
-      decoded = jsonDecode(raw);
+      decoded = jsonDecode(payload);
     } on FormatException catch (e) {
-      return _userErr(req.id, 'invalid JSON in $file: ${e.message}');
+      return _userErr(req.id, 'invalid JSON in the icon payload: ${e.message}');
     }
     if (decoded is! List) return _userErr(req.id, 'icon metadata must be a JSON array of entries');
     for (final item in decoded) {