Fix delivery report callback: librdkafka passes the message by pointer

[stellarthemes]

librdkafka invokes the delivery report callback as

void dr_msg_cb(rd_kafka_t *rk, const rd_kafka_message_t *rkmessage, void *opaque);

but the binding declares the callback as taking Message by value:

fun conf_set_dr_msg_cb = rd_kafka_conf_set_dr_msg_cb(conf : ConfHandle, cb : (KafkaHandle, Message, Void*) ->)

On x86-64 Linux, a struct this size passed by value is expected in stack memory, so the callback ends up reading memory the caller never wrote — every field in the message is garbage. That's the garbled payload in #2.

On Apple Silicon, a large by-value struct is passed by reference instead, which happens to line up exactly with librdkafka passing a pointer. So the same code looks fine on a Mac — which is why #2 only reproduces on Linux servers.

Changes:

• the callback now takes Message* and reads fields through .value
• the payload string is built with the message's len — the payload is not NUL-terminated, so String.new(pointer) could over-read even with the calling convention fixed
• when err is set, log at error level instead — librdkafka reuses the payload field for the error string in that case

Checked the struct layout against librdkafka 1.8 headers; unit specs pass. Note this changes the callback type, so anyone who registered their own delivery callback with the old signature will need the same one-line change — probably worth a version bump.

Fixes #2.

[stufro]

LGMT

[stellarthemes]

do i need to do anything to get this merged?