BsonCodecs

Manual codecs for BSON-native types with no natural JSON/CBOR representation. Each detects whether it is running against the BSON backend (via the BsonExtOutput/BsonExtInput opportunistic capability the BSON value-position reader/writer implements) and uses the native BSON type tag there; on every other backend it falls back to a plain string (or, for BsonRegex, a two-field object) encoding.