PINK: fix last c_char_p temporary in set_slot_json

Completes the ctypes lifetime audit. All eight FFI call sites now
assign _to_rust_bytes() to a local var before passing to c_char_p,
ensuring the bytes object lives for the full duration of the Rust call.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prod/clean_arch/dita_v2/rust_backend.py

@@ -219,7 +219,8 @@ class _RustKernelLib:
         return json.loads(self._take_string(raw))
 
     def set_slot_json(self, handle: ctypes.c_void_p, slot_id: int, payload: Dict[str, Any]) -> None:
-        rc = self.lib.dita_kernel_set_slot_json(handle, ctypes.c_size_t(slot_id), ctypes.c_char_p(_to_rust_bytes(payload)))
+        _pb = _to_rust_bytes(payload)
+        rc = self.lib.dita_kernel_set_slot_json(handle, ctypes.c_size_t(slot_id), ctypes.c_char_p(_pb))
         if rc != 0:
             raise RuntimeError(f"dita_kernel_set_slot_json failed rc={rc}")