Still live and unfiled: an audit purge leaves a deleted `.txnlog` mmapped in the live process, reclaimed only

From dispatch finding (task `qa-wave-2026072900`):

- F-250 — **Still live and unfiled: an audit purge leaves a deleted `.txnlog` mmapped in the live process, reclaimed only at restart — the harper#846 shape.** Independently reproduced this wave by QA-853: the count-first oracle fails `strictEqual(point2.txnlogUnderRoot.length, 0)` → `1 !== 0` on `80ef45996`, catching a 16 MiB mapping of an unlinked `.txnlog` with no matching `/proc//fd` entry. The byte magnitude remains unmeasured (no reachable platform can `statSync` through `/proc//map_files`; the 16 MiB is an address range, an upper bound) — that is QA-851's job and does not weaken the count result. Its anchor is banked RED as P-606 and cannot be promoted until this is fixed.