Refactor kdb adapter into separate modules

Split kdb.rs (610 lines) into:
- kdb/mod.rs: shared types (KdbConnection, KdbCredentials, KdbError)
- kdb/read.rs: KdbDeserialize trait, kdb_read(), helper functions
- kdb/write.rs: KdbSerialize trait, kdb_write(), KdbWriteOperators

Public API unchanged.

Author: 0-jake-0