Per-subscription delivery ordering for pubsub events.

A publisher stamps every fact with a pool-monotonic seq (macula_client), contiguous within a publisher's lifetime (a refused publish does not burn a number). The mesh sends copies down several links at once, so a subscriber receives seqs out of order and macula_client dedups to the FIRST arrival — which scrambles a single publisher's stream. This module puts the deduped stream back into per-publisher order, offering three delivery modes a subscriber picks at subscribe time:

• ordered (default) — per-publisher FIFO by seq. Out-of-order arrivals are buffered and released in order; a genuinely missing seq is skipped after a timeout (see flush/3), trading a bounded delay for the lost fact.
• latest_only — deliver only if the seq exceeds the highest already delivered for that publisher (drop stale). No buffering, no head-of-line delay. For state-snapshot consumers that want freshness over completeness.
• as_arrives — deliver immediately in arrival order (the pre-1.x behaviour). Zero added latency; the consumer handles ordering itself.

seq re-bases to wall-clock microseconds when a publisher's pool restarts, so a large forward jump (> ?EPOCH_JUMP) is read as a restart: the old expected counter is abandoned rather than waited on.

Pure and side-effect-free: it returns the events to deliver now, and the caller does the sending. flush/3 is driven by the caller on a timer to release buffers whose gap has timed out.