rebuild_eras() re-digested EVERY month from scratch on every coherence pass,
including old months whose sessions never change — ~17 redundant 32B calls per pass
(a big slice of the ~40-min consolidation grind + MI50 heat). Now it compares each
month's current session count to the stored era and only rebuilds changed months
(force=True still does all). Report gains built/skipped counts.
test_era.py: builds all first pass, skips unchanged, rebuilds only a month that
gained a session, force rebuilds all. Suite 99 green, ruff clean.
(Profile rebuild re-reading all 851 sessions every pass is the bigger remaining
hog — separate, harder fix.)
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
