fix(physics): port retail stair edge backprobe

docs/research/2026-07-31-271-stair-side-slide-capture.md

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+# #271 — Stair-side uphill reversal capture
+
+**Date:** 2026-07-31
+
+**Status:** closed; retail control flow restored and user live gate passed
+
+## Symptom
+
+When the local player ran diagonally uphill while pressing into the side of
+an outdoor staircase, the character could suddenly move backward and rapidly
+slide to the bottom. The symptom was intermittent because it required the
+forward candidate to hit the side wall while the step-down recovery crossed a
+tread edge.
+
+This is not an RDP, render-rate, animation, or gravity symptom. It reproduced
+inside the pure Core collision resolver from one captured input frame.
+
+## Live evidence
+
+The bounded capture is under the ignored local artifact pointer:
+
+`artifacts/issue271-stair-side/LATEST.txt`
+
+It contains 677 local-player physics quanta plus the matching resolver stream.
+The first decisive frame is quantum 310:
+
+```text
+current = (133.03775, 75.53931, 59.608147)
+target  = (133.33783, 76.42308, 59.608147)
+input   = forward + run
+result  = (133.18779, 75.19872, 59.316677)
+normal  = (-1, approximately 0, approximately 0)
+```
+
+The X side-wall collision was valid, but the tangential Y component reversed:
+an uphill request of `+0.88377` produced `-0.34059`. Three frames later,
+quantum 313 snapped from Z `59.52598` to terrain Z `58.005`. A second attempt
+reproduced the same family at quanta 479–482, falling from Z `60.96376` to
+`58.005`.
+
+## Retail oracle
+
+Named retail:
+
+- `CTransition::edge_slide` at `0x0050B3D0`
+- current-walkable branch at `0x0050B44A`
+- no-walkable back-probe at `0x0050B458–0x0050B50F`
+- `SPHEREPATH::precipice_slide` at `0x0050CC80`
+
+Retail tests only the current `SPHEREPATH::walkable` pointer. If it is null,
+retail:
+
+1. offsets the failed candidate back to the current sphere center;
+2. runs `step_down` there to rediscover the surface actually under the mover;
+3. restores the failed candidate;
+4. runs `precipice_slide` against the newly discovered polygon; or
+5. returns `COLLIDED_TS` when the back-probe found no walkable polygon.
+
+Retail has no substitution of an older saved walkable polygon in either null
+case.
+
+## ACDream divergence and root cause
+
+`EdgeSlideAfterStepDownFailed` previously called
+`SpherePath.RestoreLastWalkable()`:
+
+- before deciding whether to enter the retail back-probe; and
+- again when the back-probe found no current walkable polygon.
+
+`LastWalkable` is a separate ACDream history used by the still-open
+CliffSlide compatibility path. At a staircase side wall it could describe the
+preceding tread rather than the surface below the current player position.
+Promoting it into the current slot bypassed retail's back-probe.
+`PrecipiceSlide` then projected the failed forward candidate along the stale
+tread edge, producing the backward/downhill displacement seen in the capture.
+
+The fix removes both stale-history promotions from the edge-slide dispatch.
+Current walkable state still takes retail's direct precipice path; absent
+state now always takes retail's current-position back-probe.
+
+## Deterministic regression
+
+`Issue185OutdoorStairsSeamReplayTests` reuses the captured
+`0x01000AC5` staircase collision fixture and the exact quantum-310 position,
+contact plane, movement delta, player flags, and 1.5 m Setup step-down height.
+
+Pre-fix:
+
+```text
+out = (133.187790, 75.346481, 59.430882)
+```
+
+Fixed:
+
+```text
+out = (133.187790, 76.078186, 60.016247)
+```
+
+The regression requires meaningful positive uphill progress and forbids a
+downhill Z displacement. The complete Core Release suite passes 4,108 tests /
+2 skips; the complete Release solution passes 10,062 tests / 5 skips.
+
+## Live acceptance
+
+The user repeatedly ran uphill while pressing into both sides of the affected
+staircase. Movement remained stable and the former rapid downhill reversal did
+not recur. The client then closed through the normal logout path, with ACE
+confirming graceful logout.