fix(physics): #32 L.2c precipice edge-slide context

Port the first retail precipice-slide slice from named retail/ACE: terrain and BSP walkable hits now preserve polygon vertices, failed step-down edges back-probe to rediscover the walkable polygon, and edge-slide can run precipice/cliff slide instead of only hard-stopping.

Adds pseudocode anchors plus regression coverage for terrain polygon context and loaded-terrain boundary edge-slide.

Co-authored-by: Codex <codex@openai.com>

docs/ISSUES.md

@@ -189,19 +189,22 @@ step-down boundaries, retail often slides along the boundary. acdream still
 hard-blocks or accepts too much in several of these cases.
 
 **Root cause / status:** Tracked under Phase L.2c. Wall-adjacent
-`step_up_slide` now feels acceptable in live testing. L.2c plumbing now passes
-the retail-default `EdgeSlide` flag into local and remote movement and logs
-failed step-down edge cases behind `ACDREAM_DUMP_EDGE_SLIDE=1`. Remaining gap:
-preserve walkable polygon context for `precipice_slide` and finish
-`cliff_slide` / `NegPolyHit` dispatch. Named retail anchors include
-`CTransition::edge_slide`, `CTransition::cliff_slide`,
-`SPHEREPATH::precipice_slide`, and `SPHEREPATH::step_up_slide`.
+`step_up_slide` now feels acceptable in live testing. Local/remote movement
+passes the retail-default `EdgeSlide` flag. The first precipice-slide slice now
+preserves terrain/BSP walkable polygon vertices and runs the retail back-probe
+before `SPHEREPATH::precipice_slide`; `ACDREAM_DUMP_EDGE_SLIDE=1` now reports
+whether a failed step-down had polygon context. Remaining gaps: real-DAT
+building-edge fixtures, fuller `cliff_slide` coverage, and `NegPolyHit`
+dispatch. Named retail anchors include `CTransition::edge_slide`,
+`CTransition::cliff_slide`, `SPHEREPATH::precipice_slide`, and
+`SPHEREPATH::step_up_slide`.
 
 **Files:** `src/AcDream.Core/Physics/TransitionTypes.cs`,
 `src/AcDream.Core/Physics/BSPQuery.cs`,
 `tests/AcDream.Core.Tests/`.
 
-**Research:** `docs/plans/2026-04-29-movement-collision-conformance.md`.
+**Research:** `docs/plans/2026-04-29-movement-collision-conformance.md`,
+`docs/research/2026-04-30-precipice-slide-pseudocode.md`.
 
 **Acceptance:** Synthetic and real-DAT tests cover wall-slide, roof-edge slide,
 cliff/precipice slide, failed step-up/step-down, and the jump-clears-edge case.

docs/plans/2026-04-29-movement-collision-conformance.md

@@ -118,6 +118,13 @@ precipices.
   edge, walkable, and collision rules; jumping clears `OnWalkable` and only
   succeeds when the airborne path actually clears geometry.
 
+Current shipped slice (2026-04-30): wall-adjacent `step_up_slide` feels
+acceptable in live testing; player/remote movers pass `EdgeSlide`; terrain and
+BSP step-down/find-walkable now preserve walkable polygon vertices; failed
+step-down edge cases perform the retail back-probe before
+`SPHEREPATH::precipice_slide`. Remaining L.2c work is real-DAT building-edge
+fixtures, fuller `cliff_slide` coverage, and `NegPolyHit` dispatch.
+
 ### L.2d - Shape Fidelity: Sphere / CylSphere / Building Objects
 
 Goal: object collisions use retail shape semantics, not one simplified

docs/research/2026-04-30-precipice-slide-pseudocode.md

@@ -0,0 +1,110 @@
+# Precipice Slide Pseudocode
+
+Date: 2026-04-30
+
+Phase: L.2c - Movement & Collision Conformance
+
+## Retail Anchors
+
+- Named retail: `CTransition::edge_slide`, `acclient_2013_pseudo_c.txt:273001`
+- Named retail: `CTransition::cliff_slide`, `acclient_2013_pseudo_c.txt:272397`
+- Named retail: `SPHEREPATH::precipice_slide`, `acclient_2013_pseudo_c.txt:274316`
+- ACE cross-check: `Transition.EdgeSlide`, `Transition.CliffSlide`,
+  `SpherePath.PrecipiceSlide`
+- ACE cross-check: `Polygon.find_crossed_edge`
+
+## Edge-Slide Flow
+
+When a grounded mover has contact state but the next candidate position has no
+walkable surface within step-down reach, retail does not immediately accept the
+fall or hard-stop. It enters `CTransition::edge_slide`.
+
+```text
+edge_slide(transitionState, stepDownHeight, walkableZ):
+  if object is not OnWalkable or EdgeSlide is disabled:
+    clear walkable
+    restore candidate check position
+    clear current contact plane
+    mark cell array valid
+    transitionState = OK
+    return handled
+
+  if current collision has a contact plane below walkableZ:
+    transitionState = cliff_slide(contact plane)
+    clear walkable and restore candidate check position
+    clear current contact plane
+    return not-final
+
+  if sphere_path.walkable exists:
+    transitionState = precipice_slide()
+    clear current contact plane and restore candidate check position
+    return transitionState == Collided
+
+  if current collision has any contact plane:
+    clear walkable
+    restore candidate check position
+    clear current contact plane
+    transitionState = OK
+    return handled
+
+  move CheckPos back from failed candidate to the current sphere center
+  step_down(stepDownHeight, walkableZ) to rediscover the walkable polygon
+  clear current contact plane
+  restore the failed candidate check position
+
+  if a walkable polygon was discovered:
+    set walkable_check_pos from the candidate sphere in walkable space
+    transitionState = precipice_slide()
+    return transitionState == Collided
+
+  clear walkable
+  mark cell array valid
+  transitionState = Collided
+  return handled
+```
+
+## Precipice Slide
+
+`SPHEREPATH::precipice_slide` is the edge-normal half of edge-slide. The crucial
+input is the walkable polygon that the mover just left; without that polygon,
+there is no crossed edge to slide along.
+
+```text
+precipice_slide():
+  normal = zero
+  found = walkable.find_crossed_edge(walkable_check_pos, walkable_up, normal)
+
+  if not found:
+    clear walkable
+    return Collided
+
+  clear walkable
+  step_up = false
+
+  normal = walkable_pos.frame.LocalToGlobalVec(normal)
+
+  blockOffset = LandDefs.GetBlockOffset(curr cell, check cell)
+  movementOffset = global_sphere.center - global_curr_center.center + blockOffset
+
+  if dot(normal, movementOffset) > 0:
+    normal = -normal
+
+  return global_sphere.slide_sphere(transition, normal, global_curr_center.center)
+```
+
+## Porting Notes
+
+acdream already had the `Polygon.find_crossed_edge` math inside `BSPQuery`, but
+the live diagnostic showed `walkableValid=False` at the failed step-down edge
+branch. The port must therefore preserve or rediscover the walkable polygon,
+not just pass the `EdgeSlide` flag.
+
+For the first L.2c slice:
+
+- terrain supplies the exact current triangle vertices alongside its plane;
+- BSP step-down/find-walkable records world-space polygon vertices when the
+  caller supplies the object's world origin;
+- the failed step-down edge branch performs the retail back-probe to current
+  position before calling precipice slide;
+- `CELLARRAY`, full `cell_bsp` ownership, and cross-cell building portals remain
+  L.2e work.