docs: #137 corridor-phantom pickup prompt for the next session

Both mechanisms + evidence pointers + the done-already oracle greps,
ordered mechanism-2-first (the sliding-normal wedge, pure acdream-side)
then the mechanism-1 retail cdb trace (needs the user running retail
at the Facility Hub corridor).

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

docs/research/2026-07-06-137-corridor-phantom-pickup-prompt.md

@@ -0,0 +1,76 @@
+# Pickup prompt — #137 corridor phantom collision (paste into a fresh session)
+
+Read `claude-memory/project_physics_collision_digest.md` FIRST (binding
+DO-NOT-RETRY table), then **ISSUES #137** (the 2026-07-05 CHARACTERIZED
+section — the full evidence chain lives there), then this file. The 2026-07-05
+session's ledger for context: #172 (CCylSphere family port), #173 (remote
+ceiling bounce), #174 (motion-queue drain — doors work after jumping), #175
+(door collision at the motion-table closed pose, two takes).
+
+**The bug (user-verified repro, Facility Hub 0x8A02):** running down a
+corridor, an INVISIBLE blocker stops the player mid-corridor; the player can
+walk around it. Two stacked mechanisms, both evidence-pinned:
+
+## Mechanism 1 — PortalSide portal polygons are solid for us
+
+- Live: crossing corridor cells `0x8A02016E → 0x8A02017A` at world x≈85.25
+  recorded ONE wall hit, normal (−1,0,0) — straight against the movement
+  (`launch-175-verify2.log:42858`, worktree root).
+- Dat (`Issue137CorridorSeamInspectionTests`, committed): cell 0x8A02016E's
+  three portals to 0x011E (polys 1/3/5, flags=**PortalSide**, NOT ExactMatch)
+  are PRESENT in `CellStruct.PhysicsPolygons`; every ExactMatch portal in the
+  same cell is absent from the physics set. The cell's rotation maps those
+  portal planes into the world −X wall the player hit.
+- Oracle greps DONE (do not repeat): `CCellStruct::UnPack` 0x00533d00 loads
+  physics polys + BSP verbatim (no portal stripping);
+  `CPolygon::pos_hits_sphere`/`hits_sphere`/`polygon_hits_sphere_slow_but_sure`
+  (0x005394f0/0x00539540/0x00538a10) are pure geometry;
+  `CCellPortal` (0x0053bab0) carries portal→CPolygon + portal_side +
+  exact_match but the BSP test chain never consults it.
+- **NEXT (step −1 protocol, needs the user):** cdb-attach a live retail
+  client at this exact corridor (Facility Hub, the 016E↔011E portals) and
+  trace which path lets retail through: breakpoint
+  `BSPTREE::find_collisions` / `BSPLEAF::sphere_intersects_poly` /
+  `CPolygon::pos_hits_sphere` and see whether the portal polys are ever
+  TESTED (candidate A: sidedness/stippling — the polys carry stip=NoPos —
+  or the pos_hits_sphere tail's directional cull) or never REACHED
+  (candidate B: transit/membership order hands the sphere to the neighbor
+  cell whose geometry has a real hole). Toolchain crib:
+  `claude-memory/project_retail_debugger.md` + the CLAUDE.md
+  "Retail debugger toolchain" section. Verify the binary with
+  `py tools/pdb-extract/check_exe_pdb.py` first.
+- ⚠️ Do NOT ship a "skip portal polys in the physics BSP" filter on
+  assumption — if retail's answer is sidedness or test order, a blanket
+  skip opens holes (some PortalSide polys may be legitimately solid from
+  one side — one-way drops etc.).
+
+## Mechanism 2 — the sliding-normal absorbing wedge (fix independently)
+
+- After the single seam hit, EVERY forward resolve returns `ok=False
+  hit=no` with zero advance: the body-persisted SlidingNormal (−1,0,0)
+  projects the +X offset to exactly ZERO in `Transition.AdjustOffset`, and
+  the stepping loop's abort-small-offset fires at step 0 (TransitionTypes
+  `FindValidPosition` loop, `return i != 0 && …`) — BEFORE any collision
+  test could refresh the state. An ABSORBING wedge; strafing escapes it
+  (the user's "push through on the side").
+- Retail re-derives slide state per frame — `OBJECTINFO::get_object_info`
+  pc:279992 "governs only the NEXT frame" (#116 notes in the digest).
+  AUDIT: who writes the body's persisted SlidingNormal
+  (PhysicsEngine.ResolveWithTransition seed ~:995-1040 + the writeback),
+  and where retail CLEARS it when contact does not recur. This is the #116
+  slide-response family — check ISSUES #116 before changing anything
+  (oracle-first; the digest's DO-NOT-RETRY table applies).
+- Likely the bigger playability win: without the wedge, mechanism 1 alone
+  would be a momentary stutter, not a dead stop.
+
+**Order:** mechanism 2 first (pure acdream-side audit + fix, testable with a
+replay-style unit test: seed a body with a stale sliding normal, resolve
+forward with no obstruction in range, assert the step is NOT zeroed), then
+the mechanism-1 cdb session when the user can run retail side-by-side.
+
+**Gotchas:** PowerShell Tee logs are UTF-16 (`tr -d '\000'` before grep);
+the user manages client lifecycle; probes RESOLVE/CELL/BUILDING are
+DebugPanel-toggleable (ACDREAM_DEVTOOLS=1); the [shape-pose] line
+(ACDREAM_DUMP_MOTION=1) prints each BSP registration's pose source.
+Register rows to touch if fixes ship: none exist yet for either mechanism —
+add per the same-commit rule; #116's row interactions per the digest.