docs(handoff): A6.P3 #98 Step 6 — issue + claude.md handoff

Final step of the apparatus plan. Updates ISSUES.md issue #98 and
CLAUDE.md's M1.5 status to reflect:

- The apparatus completed (Steps 1-5 land in commits 35b37df →
  28c282a).
- The real divergence: retail's sphere is at world Z ≈ 94.48 (resting
  on cottage floor) when find_walkable accepts; acdream's failing-
  frame sphere is 2.47m lower at world Z ≈ 92.01.
- The four fix targets, in priority order. Fix plan is the NEXT plan,
  scoped to Target 1 (step-up + ramp climb Z gain) or Target 2
  (cottage-cell sphere reference).
- The replay harness (Issue98CellarUpReplayTests) is the test loop —
  any fix that doesn't change the failing assertions is not the fix.

Today's commit graph on top of slice 5 (cf3deff):
  35b37df  triage — revert neg-poly + bldg-check experiments
  f62a873  Step 2 — cell-dump probe + roundtrip test
  3f56915  Step 2 capture — 3 real-geometry cell fixtures
  856aa78  Step 3 — deterministic replay harness (7 tests)
  6f666c1  Step 4 — retail cdb find_walkable capture script
  28c282a  Step 5 — replay vs retail divergence comparison
  (this)   Step 6 — ISSUES.md + CLAUDE.md handoff

Test baseline: 1167 + 8 (8 pre-existing failures, +19 new passing
tests across the apparatus). Build green throughout.

A6.P3 #98 is now in evidence-driven mode. Fix plan starts from the
divergence doc at
docs/research/2026-05-23-a6-p3-issue98-replay-comparison.md.

Pickup prompt for the fix-plan session is in §"Pickup prompt for the
fix plan" of that doc.

CLAUDE.md

@@ -744,16 +744,31 @@ indoor branch (point-in check against `fallbackCellId`'s CellBSP
 before falling through to FindCellList). Data confirms ping-pong is
 FULLY CLOSED — scen4 cellar capture shows 1 cell-transit (login
 teleport) vs 20+ pre-fix. **#90 workaround now redundant — deferred
-to A6.P4 removal. #98 SHARP DIAGNOSIS LANDED 2026-05-22** (commits
-`134c9b8` retail capture + `efb5f2c` issue update): paired retail+acdream
-cdb captures show our BSPQuery.FindCollisions picks Path 5 (Contact
-step_up push-back) for the cellar ramp polygon when retail picks
-Path 6 (find_walkable land). Cellar-up bug is NOT cell-resolver, NOT
-walk_interp, NOT dat-fidelity — it's BSP path-selection. Handoff doc
-at [`docs/research/2026-05-22-a6-p3-handoff.md`](docs/research/2026-05-22-a6-p3-handoff.md)
-with concrete next-session steps + pickup prompt. Current A6 phase:
-**A6.P3 slice 4 — fix issue #98 BSP path-selection in
-BSPQuery.FindCollisions dispatcher.**
+to A6.P4 removal. #98 APPARATUS COMPLETE 2026-05-23 evening**
+(commits `35b37df` triage → `f62a873` cell-dump probe → `3f56915`
+fixtures → `856aa78` replay harness → `6f666c1` cdb script →
+`28c282a` divergence comparison doc). Four sessions of speculative
+fixes (10+ variants) shipped the wrong diagnosis each time; this
+session shipped the APPARATUS that turns evidence-driven analysis
+into a 200ms test loop. Real divergence: retail's sphere is at
+world Z ≈ 94.48 (resting on cottage floor) when find_walkable
+accepts; acdream's failing-frame sphere is at world Z ≈ 92.01
+(2.47m lower). Retail's ContactPlane writes during cellar-up are
+ONLY flat floors (cellar floor or cottage floor), never the ramp.
+Retail's find_crossed_edge fires once in 35K BPs; ours uses it
+heavily. **Fix targets (priority): (1) Transition.AdjustOffset
+slope projection / DoStepUp WalkInterp handling — ramp climb
+doesn't gain Z; (2) cottage-cell candidacy using wrong sphere
+reference; (3) find_crossed_edge over-use; (4) ramp polygon normal
+divergence (low confidence).** Full divergence reading +
+fix-plan pickup prompt at
+[`docs/research/2026-05-23-a6-p3-issue98-replay-comparison.md`](docs/research/2026-05-23-a6-p3-issue98-replay-comparison.md).
+Current A6 phase:
+**A6.P3 — write evidence-driven fix plan against Target 1 or 2;
+NO speculative fixes (4 prior sessions guessed and were wrong).
+The replay harness ([`tests/AcDream.Core.Tests/Physics/Issue98CellarUpReplayTests.cs`](tests/AcDream.Core.Tests/Physics/Issue98CellarUpReplayTests.cs))
+is the test loop; a fix that doesn't change the failing assertions
+is not the fix.**
 Findings doc:
 [`docs/research/2026-05-21-a6-cdb-capture-findings.md`](docs/research/2026-05-21-a6-cdb-capture-findings.md).
 Original demo scenario (Holtburg Sewer end-to-end) is unreachable: sewer