Measured while building #16: at 10 000 enemies the grid rebuild costs
0.057 ms against 0.021 ms for move_enemies — 2.7x more, not less. ADR-0004
§3 counted arithmetic and missed what dominates: move_enemies is a linear
table walk, the rebuild writes into ~7000 scattered buckets.
The decision does not change. It never rested on the comparison but on the
absolute bound of 0.10 ms, and 0.057 ms is 0.34 % of the frame budget; an
incremental update would pay the same scattered write on every cell
crossing. Revision trigger 1 and the acceptance criterion of spec feature 3
are restated on that bound, which is what the runnable check now asserts.
Also records the second deviation the build produced: a cell holds
(Entity, Vec2) rather than Entity, because soft separation writes Transform
while reading its neighbours' positions.
