Closes#96
Replaces the per-field `json.dumps(value)` probe — which fully serialized
and discarded the result just to check serializability — with a type-check
fast path using `isinstance()`. The `_SERIALIZABLE_TYPES` tuple is defined
at module level so it's not recreated on every log call.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>