Enemies spawn on the arena rim, walk radially at the tower and stop at the
attack range. The spawner is cap-driven — it tops the field up to the target
population instead of placing N enemies on a schedule — so the target
population is the binding quantity the difficulty curve will stand on.
The spatial grid of ADR-0004 lands with them, because it now has its first
consumer and #17 needs it unchanged for target selection and projectile
collision. One full rebuild per frame is the only pass over the enemy set;
every consumer asks the grid.
Soft separation is a clamped positional offset, not a force: overlap stays
allowed, and capping the offset below one movement step is what keeps the
tower ring from pulsing.
Closes#16.
