Both gate-admitted census tests re-scanned the whole ~1200-file corpus
redundantly: legacy-column-literal-census ran census() twice in full
(~900ms each) and no-legacy-move-targets re-read the entire tree a second
time in its anti-vacuity case. Memoize the census result and share a
comment-stripped read cache so each tree is scanned once per run.
Fusion-Task-Id: none
71b13e7888