Export typed CodecError surface (discriminated union / ERROR_CODES) for structured consumer error handling

[ignromanov]

README documents 14 error variants but JS consumers can only string-match .message. Error messages are semver-locked per REGISTRY.md:53-59, making string matching a viable but brittle workaround.

Additive and non-breaking to ship in a 0.1.x patch: expose a typed discriminated union (e.g. CodecError) or an ERROR_CODES map so consumers can catch and discriminate errors structurally rather than via .message pattern matching.