mind 5.1.0 — soak-hardened forgetting

A 180-day simulated-usage soak test (real code, injected clock, now in CI) caught two calibration bugs unit tests couldn't: monthly-cadence facts died one day before their first recall, and decayed weight vetoed exact matches. Fixed with a 45-day grace window, weight-floor ranking, ~2-weeks-per-recall stability, and archive-on-prune (nothing is ever destroyed). Day-180 results: core-fact survival 15/15 (daily/weekly/monthly), stale junk surviving 0/256, graph bounded, 0.37 ms recall on the aged graph. 61 tests.