Companion to the route/phase viz: for every free
configuration-space node, precomputed offline (build_level2_navigation_field.py),
which single step shortens its A*-cost distance to the backbone route -- so a live reconnect
after a hazard dodge is an array lookup instead of a fresh full-map A* search. Fields are keyed
by mission phase; the two phases marked fallback below have
no field (dynamic boss-arena geometry / a revisited retreat-cross-advance sub-mission) and keep
today's live-only navigation, shown hatched.