fix #137 (seam shake): CheckOtherCells queried remaining cells at a stale pre-climb center

The P2 cellar-lip lesson one loop deeper. CheckOtherCells takes footCenter
by value and used it for every cell in the loop — but a mid-loop query can
MOVE the sphere: at the Facility Hub cell boundaries the neighbor's ramp
floor full-hits, step_sphere_up climbs the foot +0.6mm and returns OK, and
the loop continued querying the REMAINING cells (including the under-room,
portal-ring-3) at the pre-climb height — 0.4mm inside the double-faced
floor slab, grazing its underside (the under-room's ceiling) within the
near-miss window. That dispatched a neg-poly step-up with a DOWNWARD
normal, whose failure funneled into slide_sphere's opposing branch ->
synthetic reversed-movement collision -> Collided -> revert, every frame:
the seam shake (and, pre-mechanism-2-fix, the original absorbing wedge's
entry).

Retail check_other_cells reads the LIVE sphere_path.global_sphere for
every cell (each cell's find_collisions receives the transition itself,
pc:272717+). Fix: re-read footCenter = sp.GlobalSphere[0].Origin at the
top of each loop iteration.

All three Issue137CorridorSeamReplayTests repros un-skipped: the
snapshot-exact west-boundary crossing (capture tick 4101), the east
deep-straddle, and the clean-run lifecycle all GREEN. Full suites: Core
2556 / App 713 / UI 425 / Net 385, 0 failures.

Visual gate pending: corridor run + the purple seam flashing re-check
(expected to be the render exposing the same per-frame oscillation).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ISSUES.md

@@ -1183,6 +1183,20 @@ walls** in particular. (Symptoms not fully characterized yet: likely walking thr
 openings that should block / blocking at openings that should pass, and door collision not
 matching the door's open/closed state.)
 
+**SEAM SHAKE FIXED (same day): the stale `footCenter` in `CheckOtherCells`'
+per-cell loop** — the P2 cellar-lip lesson one loop deeper. A mid-loop
+other-cell query can MOVE the sphere (the boundary full-hit dispatches
+step_sphere_up; the successful climb lifts the foot +0.6 mm and returns
+OK), and the remaining cells were then queried with the by-value
+pre-climb center — 0.4 mm inside the floor slab, grazing the under-room's
+ceiling and firing the chain below. Retail's `check_other_cells` reads the
+LIVE `sphere_path.global_sphere` per cell (pc:272717+). Fix: re-read
+`footCenter = sp.GlobalSphere[0].Origin` per iteration. All three
+`Issue137CorridorSeamReplayTests` repros un-skipped and GREEN; full suites
+green. Visual gate pending. (The step 3 "restore clobbers CheckPos" wording
+below was the right CLASS but the wrong site — CheckPos was fine; the
+stale copy was the loop's captured parameter.)
+
 **GATE 2026-07-06 FAILED — THIRD MECHANISM CHARACTERIZED (the seam shake),
 deterministic offline repro secured:** with mechanisms 1+2 fixed the dead
 stop became a SHAKE at cell seams (+ purple floor flashing there — almost

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -1804,6 +1804,21 @@ public sealed class Transition
         {
             if (cellId == sp.CheckCellId) continue;
 
+            // #137 seam shake (2026-07-06): a mid-loop query can MOVE the
+            // sphere — a Path-5 full hit dispatches step_sphere_up, and a
+            // successful climb lifts the foot (the 0.6 mm ramp-slab lift at
+            // the Facility Hub boundaries) yet returns OK, so the loop
+            // continues. Retail's check_other_cells reads the LIVE
+            // sphere_path.global_sphere for every cell (each cell's
+            // find_collisions receives the transition itself, pc:272717+);
+            // querying the remaining cells with the pre-climb center re-tests
+            // the boundary ~0.4 mm inside the floor slab and grazes the
+            // under-room's CEILING (the slab underside) — the neg-poly
+            // step-up-with-a-downward-normal chain that shook the player at
+            // every corridor seam. Same lesson as the P2 cellar-lip fix
+            // (RunCheckOtherCellsAndAdvance's entry refresh), one loop deeper.
+            footCenter = sp.GlobalSphere[0].Origin;
+
             if ((cellId & 0xFFFFu) < 0x0100u)
             {
                 var terrainWalkable = engine.SampleTerrainWalkable(footCenter.X, footCenter.Y);

tests/AcDream.Core.Tests/Physics/Issue137CorridorSeamReplayTests.cs

@@ -168,7 +168,7 @@ public class Issue137CorridorSeamReplayTests
     /// opposing-normals branch → Collided → revert. Repeat.
     /// </para>
     /// </summary>
-    [Fact(Skip = "#137 seam shake — deterministic RED repro; the post-step-up check-pos restore clobbers the lifted position (P2 stale-snapshot class). Un-skip with the fix.")]
+    [Fact]
     public void SeamShake_WestBoundary_SnapshotExact_Advances()
     {
         var datDir = FindDatDir();
@@ -255,7 +255,7 @@ public class Issue137CorridorSeamReplayTests
     /// frame (the "shaking at the seam" report). The deeper straddle start is
     /// what the original replay frame (84.638 → 85.253) didn't cover.
     /// </summary>
-    [Fact(Skip = "#137 seam shake — deterministic RED repro; the post-step-up check-pos restore clobbers the lifted position (P2 stale-snapshot class). Un-skip with the fix.")]
+    [Fact]
     public void SeamCrossing_FromDeepStraddleStart_Advances()
     {
         var datDir = FindDatDir();
@@ -285,7 +285,7 @@ public class Issue137CorridorSeamReplayTests
             $"reversed-movement normal = the 2026-07-06 seam shake.");
     }
 
-    [Fact(Skip = "#137 seam shake — deterministic RED repro; the post-step-up check-pos restore clobbers the lifted position (P2 stale-snapshot class). Un-skip with the fix.")]
+    [Fact]
     public void SeamCrossing_DoesNotPersistSyntheticSlidingNormal_AndRunContinues()
     {
         var datDir = FindDatDir();