fix(environment): own JSON fields across capture, state and replay

docs/agent-environments.md

@@ -871,8 +871,9 @@ names a `document` view once, at its initial document's `path`. The `version`,
 and the `state_evidence_id` and `types` that `data/get` reports, must match
 the served state, so a `data/get` result loads back unchanged. A number without
 an exact native value, or whose serialized spelling would not be read as exact
-(an integer from 2^53 below 1e21, such as `1e20`, in any spelling), or a
-`__proto__` member is refused rather than altered.
+(an integer from 2^53 below 1e21, such as `1e20`, in any spelling), is refused
+rather than altered. Caller data preserves all own JSON members, including
+`__proto__`; unknown members of the state document's structure are refused.
 An invalid document is `-32602` naming the first problem by JSON pointer, as in
 `/rea_state/entities/3/collection` (also in `error.data.path`); a problem
 inside caller data (`fields`, `value`, `types`) is located at that member, so