fix(physics): port retail stair edge backprobe

docs/ISSUES.md

@@ -98,6 +98,38 @@ Copy this block when adding a new issue:
 
 ---
 
+## #271 — Stair-side collision reverses uphill movement and rapidly slides the player down
+
+**Status:** DONE — 2026-07-31 (implementation + user live gate)
+**Severity:** MEDIUM (movement feel and navigation)
+**Component:** physics / `edge_slide` / `precipice_slide`
+
+**Symptom:** while running diagonally up an outdoor staircase and pressing
+against its side, the character could suddenly move backward and slide rapidly
+to the bottom.
+
+**Root cause:** a 677-quantum live trace caught the first bad frame. A valid
+X side-wall collision turned a requested `+0.88377` uphill Y displacement into
+`-0.34059`, followed three frames later by a 1.52 m snap to terrain.
+`EdgeSlideAfterStepDownFailed` promoted ACDream's separately retained
+`LastWalkable` polygon into the current `SPHEREPATH::walkable` slot. At the
+stair side this could be the preceding tread, so `PrecipiceSlide` projected
+against stale geometry and reversed the tangent.
+
+Named-retail `CTransition::edge_slide @ 0x0050B3D0` never substitutes an older
+polygon: when current `walkable` is null it back-probes at the current sphere
+center, restores the failed candidate, and only then invokes
+`SPHEREPATH::precipice_slide @ 0x0050CC80`. Both stale-history substitutions
+are removed. The exact captured frame is pinned against the installed stair
+fixture: pre-fix output moved downhill to Y `75.346481`, while the retail-flow
+result advances uphill to Y `76.078186` and Z `60.016247`. Core Release passes
+4,108 tests / 2 skips; the complete Release solution passes 10,062 tests /
+5 skips. The user then repeatedly climbed the same stairs while pressing into
+their sides and confirmed the rapid downhill slide was gone. Evidence:
+`docs/research/2026-07-31-271-stair-side-slide-capture.md`.
+
+---
+
 ## #270 — Stuck spell animations + intermittently missing monster attack animations
 
 **Status:** CLOSED 2026-07-31 — both symptoms user-verified fixed (stuck casts: exhaustion-edge gate `a46c8e65`; missing monster attack animations: spawn settle placement `21b3a3f3` + lost-cell retry `807fdb5f`). Final settle-session log: 14/15 spawn settles grounded; Falling-refusal spam 2,954 → 15 transient pre-settle lines. All #270 probes stripped.

docs/plans/2026-07-29-physics-parity-campaign.md

@@ -389,6 +389,16 @@ root-caused, retail-ported, and user-accepted in the same session:
   are now ported, focused/full gates pass, and the user accepted repeated
   slope jumps. Evidence:
   `docs/research/2026-07-31-269-slope-stop-capture.md`.
+- **#271 closed 2026-07-31** — a bounded stair-side
+  trace proved ACDream could bypass retail's current-position edge back-probe
+  by promoting a stale `LastWalkable` tread. That made PrecipiceSlide reverse
+  an uphill tangent and rapidly carry the player down the stairs. The two
+  stale-history substitutions are removed; current-walkable, back-probe, and
+  no-walkable outcomes now follow `CTransition::edge_slide @ 0x0050B3D0`.
+  The exact captured frame is pinned in the existing installed-stair fixture
+  and the complete Release suite passes 10,062 tests / 5 skips. The user
+  accepted repeated uphill runs while pressing into the stair sides. Evidence:
+  `docs/research/2026-07-31-271-stair-side-slide-capture.md`.
 
 Matrix rows accepted so far: speed parity, roof slide, downhill bounce,
 flat pop, uphill landing, and #269's slope-stop feel

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

@@ -0,0 +1,108 @@
+# #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.