0395 — The pending (head, xmax) pair is stored unconditionally on every poll

Context. The snapshot fence proves a (head, xmax) pair out when a later statement’s snapshot xmin passes its xmax. Under nonstop producer traffic, a fresh pair never proves out inside its own poll — some transaction always overlaps — so proof must come from a pair persisted by an earlier poll.

Decision. The fresh pair writes to pending_head/pending_xmax unconditionally, not only when it fails to prove immediately. If storing were conditional, the next poll could find nothing to prove against and claims would stall behind traffic forever.

Consequences. Every claiming poll carries the small cursor-row write. Verified with an 8-producer sustained-load harness: progress while producing, 14k/14k messages drained, no stall.