Replace the A6.P4 Collided shortcut in transitional_insert's neg_step_up==0
branch with the faithful CSphere::slide_sphere. Retail
(acclient_2013_pseudo_c.txt:273350-273351, CTransition::transitional_insert):
neg_step_up==0 (HEAD-sphere near-miss) -> slide_sphere -> continue the insert
loop; neg_step_up==1 (foot) -> step_up_slide. The acdream foot branch already
did that; only the head branch took the shortcut (SetCollisionNormal + return
Collided = dead hard-stop). The slide itself is the existing
SlideSphereInternal (Sphere.SlideSphere port): it strips the into-wall
component and keeps the tangential crease (collisionNormal x contactPlane.N).
Surfaced by the B1 near-miss-gate fix (abbd761): once the grounded mover climbs
onto the cottage floor, its head sphere brushes the cellar stairwell walls and
the old hard-stop wedged it (2026-06-04 live capture: 274 (0,-1,0) + 78
(1,0,0) hits, out==current, dead oscillation). Post-fix capture shows 96
hit-and-advanced frames (the body slides along the walls).
Visual-verified 2026-06-04: closed cottage door still BLOCKS (no walkthrough --
drifts sideways along it, retail-faithful); cellar ascent now works (was always
stuck). An intermittent corner-wedge (slide into the -Y/+X wall corner) remains
-- separate finer issue, under investigation.
Core 1310 pass / 4 fail (pre-existing: 3 door documents-the-bug + D4 airborne).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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