The nether-portal section precheck from #661 answered "does this section's
palette hold a portal?" per section, per tick. Keep the answer per resident
chunk in `portal_sections`, keyed by a weak handle: an edit or a chunk
replacement rebuilds it, and it is dropped when a chunk unloads and cleared
when the block facts change, so a portal block always forces the sweep. The
entity sweep now finds portal blocks without the per-tick map.
This is a scan microbenchmark, not a whole-game claim: steady improves 2.4x
to 17x over the fixtures and the run parity-checks the cached and per-tick
answers. Rescanning every chunk every tick (cold, reload, edit_all) is slower
because a miss reads all 24 sections of a chunk; see
docs/portal-scan-benchmark.md.
rdbtCVS