diff --git a/docs/research/2026-06-03-p0-conformance-apparatus-notes.md b/docs/research/2026-06-03-p0-conformance-apparatus-notes.md
index 53bff8e..971f973 100644
--- a/docs/research/2026-06-03-p0-conformance-apparatus-notes.md
+++ b/docs/research/2026-06-03-p0-conformance-apparatus-notes.md
@@ -110,6 +110,29 @@ boundary. (Plus intrinsic building entry A3 + uniform `find_env_collisions` B1.)
 documents-the-bug test `FindCellList_DoorwayThreshold_DivergesFromRetail_PendingP1` PASSES while
 this diverges and FAILS when P1 lands → rewrite it to assert the full sequence then.
 
+**P1 design nuances (from reading `find_transit_cells` @ pc:309968 + the acdream membership map):**
+1. `find_transit_cells` (sphere variant) STRUCTURE: per portal —
+   (a) exit portal (`other_cell_id == 0xffffffff`): if a sphere crosses the exit-portal polygon
+   plane toward outside → set `exitOutside`; (b) interior/building portal with a LOADED neighbour:
+   if a sphere `CCellStruct::sphere_intersects_cell(neighbour) != OUTSIDE` → `add_cell(neighbour)`;
+   (c) interior/building portal with an UNLOADED neighbour: if a sphere crosses the portal POLYGON
+   plane (`dist` vs ±(radius+0.0002), honoring `exact_match`) → `add_cell(other_cell_id)`. After
+   all portals, `if (exitOutside) add_all_outside_cells`. The transition's `curr_cell` then advances
+   to the crossed/overlapped neighbour — BEFORE foot-containment.
+2. **acdream already has `CellTransit.FindTransitCellsSphere`** (a partial port of exactly this:
+   exit→exitOutside, loaded-neighbour→`SphereIntersectsCellBsp`, unloaded→plane-distance). So P1 is
+   NOT writing find_transit_cells from scratch — it is making `curr_cell` ADVANCEMENT use the
+   portal-crossing result (the candidate the sphere crossed into) as the membership answer, instead
+   of `FindCellSet`/`FindCellList`'s point-in-cell interior-wins pick. Audit `RunCheckOtherCellsAndAdvance`
+   + `SetCheckPos` (the swept-cell advance) — that is where the pick criterion gets chosen.
+3. **Test against the PRODUCTION path, not bare `FindCellList`.** The P0 documents-the-bug test
+   calls `FindCellList` directly (unit-level). P1's conformance must replay the golden POSITIONS
+   through `PhysicsEngine.ResolveWithTransition` (a trajectory, like `CellarUpTrajectoryReplay`,
+   seeded incl. the outdoor landcell `0031`) and assert the swept `CellId` sequence matches the
+   captured retail `change_cell` sequence. Bare-`FindCellList` is the unit pin; the trajectory is
+   the integration gate. (Needs the outdoor landcell + building portal loaded — more than the
+   building-only cache P0 uses.)
+
 ## P0 status / P1-entry checklist — COMPLETE
 
 **Apparatus: COMPLETE + GREEN.** (Conformance suite 59 pass / 1 skip / 0 fail.)