Stop bodies blinking on their way in

They spawn BEHIND the far baseline, at negative z, and the frames that step off
distance walked used a plain %, which keeps the sign of the dividend:
floor(-1.5 / 0.25) % 4 is -2, so the pick asked for 'z_g-1'. drawStatic draws
nothing for a sprite that does not exist, so the body flickered until it walked
past z=0 — worst on exactly the ones Oliver noticed, because only the heavy
bodies step off distance. The chapter boss had it too, spawning at z=-1.2 and
falling through to an undefined frame name.

One positive-modulo helper at the four picks. Verified across the whole approach
from z=-1.6: every frame resolves to a real sprite now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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