IntSpscRingBuffer

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

Layout: a single Array[Long]. Each slot packs both the data and the occupancy/control signal into one 64-bit word:

• bits 32..63: tag (0 = empty, 1 = data, 2 = done)
• bits 0..31: the user's Int value (only meaningful when tag == 1)

This makes the slot's own value act as the FastFlow signal: every insert/remove is a single memory-ordered op on a single cache line. Zero is the empty marker because the array is zero-initialized; a stored value with a non-zero tag is always non-zero regardless of the Int payload (including 0, Int.MinValue, Int.MaxValue, etc.).

In-band "done" sentinel: producers can call offerDone() to insert an end-of-stream marker after the last data value. Consumers reading via pollPacked() receive EMPTY_PACKED for empty, DONE_PACKED for the sentinel, or a TAG_FULL | Int payload value for data. This removes the need for a separate control queue alongside the data queue.

SPSC: one producer thread, one consumer thread.