Implements the kmsvc: key schema (inflight, pending/watermark keyed by
shard+partition, vis_index, dedup, fifo_lock, queue meta, shard map) plus
the atomic reap/ack Lua scripts used for safe multi-replica redelivery
and DLQ routing.
Adds queue_service.proto (SendMessage, SendMessageBatch, ReceiveMessage,
DeleteMessage, DeleteMessageBatch, ChangeMessageVisibility(Batch)) with
grpc-gateway REST annotations, plus the generated Go server/client and
gateway stubs.