The vertical arrival error was computed with CAPSULE_HALF_HEIGHT (0.5),
placing code-feet at body_y - 0.5 = 0.4 on a floor-level player.
Step surfaces at Y=0.3 are only 0.1 m away from that reference, and
Jolt can nudge the body down to ~0.8 when the bottom hemisphere contacts
the step edge, making code-feet = 0.3 and vert_err = 0 — triggering
immediate arrival while the player is still at floor level.
Fix: pass CAPSULE_HALF_HEIGHT + PLAYER_CAPSULE_RADIUS (= 0.9) to
vertical_arrival_error so actual feet = body_y - 0.9 = 0.0 on the
floor. Step goals (Y=0.3) now give vert_err=0.3 >> VERT_ARRIVE_EPS,
preventing premature arrival. Arrival fires correctly once the step
assist lifts the player onto the surface (body_y=1.2, err=0).
Add two tests pinning the new pre/post-climb behaviour.