A caller-supplied `data` with a circular reference or BigInt makes plain
JSON.stringify throw inside the file-write catch, flipping `fileBroken` and
killing ALL file logging for the session. Add `safeStringify` (WeakSet circular
guard, BigInt -> `${n}n`, wrapped to never throw) and use it for entry
serialization, so a bad payload degrades to a placeholder instead of breaking
the sink. Also model LogLevel schema-first via Schema.Literals + inferred type
(matches boundary/schema/GatewayEvent.ts), and add focused safeStringify +
poison-payload tests.
af82979d43