Constant MAX_FIELD_BYTES 

const MAX_FIELD_BYTES: usize = _; // 8_192usize

Per-field cap. The renderer can be compromised (supporter custom CSS is a real injection surface), so we treat the message/stack as attacker-controlled and refuse to log more than ~8 KiB per field. Real stack traces are well under this; a 4 GiB stack on a tight loop is a disk-fill DoS.