LongSpscRingBuffer

High-performance non-blocking SPSC ring buffer specialized for Long.

Layout: a single Array[Long]. Since every 64-bit pattern is a legal Long, the FastFlow signal requires reserving sentinel values: EMPTY (Long.MinValue) means "slot empty" and DONE (Long.MinValue + 1L) is the in-band end-of-stream marker. The array is pre-filled with EMPTY at construction.

offer(EMPTY) and offer(DONE) throw IllegalArgumentException (the same shape of contract as SpscRingBuffer.offer(null)). This costs two bit patterns out of 2^64 in exchange for a single memory-ordered op per insert/remove on a single cache line per slot, and removes the need for a separate control queue.

In-band "done" sentinel: producers can call offerDone() to insert the end-of-stream marker after the last data value. Consumers reading via pollPacked() receive EMPTY for empty, DONE for the sentinel, or the raw Long payload for data.

SPSC: one producer thread, one consumer thread.