with_codec should reject array OID collisions

[SeanTAllen]

with_array_type rejects array_oid values that collide with registered scalar codecs (_text_codecs and _binary_codecs), but with_codec doesn't perform the reverse check against array OIDs. This produces silently unreachable dead codecs in two scenarios:

1. Built-in array OID as scalar codec: CodecRegistry.with_codec(1007, SomeCodec)? succeeds because OID 1007 isn't in _text_codecs or _binary_codecs (it's handled via _ArrayOidMap). But decode() always takes the array path first for OID 1007, so the scalar codec is never reached.

2. Custom array OID as scalar codec: CodecRegistry.with_array_type(9000, 600)?.with_codec(9000, SomeCodec)? succeeds because OID 9000 is in _custom_array_element_oids, not in the codec maps. Again, decode() takes the array path first.

with_codec should check _ArrayOidMap.is_array_oid(oid) and _custom_array_element_oids.contains(oid) and error on collision, matching the symmetry of with_array_type.

Found during review of #172.