ship(phys): A6.P3 slice 3 — cell-resolver ping-pong CLOSED + #98 re-diagnosed

Slice 3 v2 (3e140cf) added point-in cell-stickiness in
ResolveCellId's indoor branch. User verification + slice3v2 capture
confirms: cell-resolver ping-pong is FULLY CLOSED.

Data:
- scen2_v2 capture (pre-slice-3): 20+ cell-transit events with
  rampant ping-pong (0xA9B4014B ↔ 0xA9B4014A ↔ 0xA9B4013F at the
  cellar boundary, Z stable ~96.4 — same tick re-classification)
- slice3v2 capture (post-fix): 1 cell-transit event (login teleport
  only) — ping-pong fully eliminated

Findings:
- A6.P2 Finding 3 (cell-resolver sling-out family) CLOSED.
- Issue #90 (sphere-overlap stickiness workaround in same function)
  now redundant; can be removed in A6.P4 after broader visual
  verification.
- Issue #97 (phantom collisions + fall-through on 2nd floor) hypothesis
  pending: same instability family, likely closed as side-effect of
  this fix. Re-test on next happy-test session.
- Issue #98 (cellar-up stuck) PERSISTS but with NEW DIAGNOSIS.
  Originally filed as cell-resolver ping-pong (which was true and now
  fixed), but user verification shows the cellar-up symptom remains
  with a DIFFERENT root cause: BSP step-physics at the cellar stair
  TOP. Push-back trace from slice3v2 capture:
    n=(0, -0.719, 0.695) sloped face (walkable per FloorZ=0.664)
    delta=(0, 0, 0.75) step-down probe lifts sphere by 0.75m
    winterp=1.0->0.0 entire walk-interp consumed per tick
  Player progresses up most of the stairs but blocks at top step
  where the cellar transitions to the cottage main floor. #98 issue
  updated with this re-diagnosis.

Includes:
- scen4_cottage_cellar_slice3 acdream.log (slice 3 v1 evidence;
  ping-pong already closed by v1's sphere-overlap stickiness, but
  v1 over-corrected by holding player in cellar during legitimate
  transitions)
- scen4_cottage_cellar_slice3v2 acdream.log (slice 3 v2 evidence;
  point-in stickiness fixes the over-correction; cellar-up reveals
  the deeper BSP step-physics bug)

Docs updated:
- ISSUES.md — #98 re-diagnosed
- docs/plans/2026-04-11-roadmap.md — A6.P3 slice 3 marked SHIPPED;
  slice 4 (or A6.P4) scoped for #98 step-physics investigation
- CLAUDE.md — Currently-working-toward block updated

Test suite: 1148 pass + 8 pre-existing fail (baseline maintained).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -738,10 +738,18 @@ which BSP step_up depends on; matching retail would require deeper
 refactor of AdjustOffset). Slice 2 verification surfaced a NEW
 M1.5-blocking bug: **user cannot walk UP out of cottage cellar — stuck
 at last step due to cell-resolver ping-pong (filed as issue #98,
-Finding 3 family).** Current A6 phase: **A6.P3 slice 3 — Finding 3
-cell-resolver stickiness (issue #98 + #90 workaround removal +
-possibly closes #97)**. Retail oracle: `CObjCell::find_cell_list`
-Position-variant at `acclient_2013_pseudo_c.txt:308742-308783`.
+Finding 3 family).** **A6.P3 slice 3 SHIPPED 2026-05-22** (commits `8898166` v1 +
+`3e140cf` v2): cell-resolver stickiness added in `ResolveCellId`'s
+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. #97 phantom collisions hypothesis pending re-test.
+#98 cellar-up symptom PERSISTS with a NEW diagnosis** (re-filed in
+#98 as BSP step physics issue at top of cellar stair flight; player
+hits sloped step-face, push-back consumes all walk-interp; not
+cell-resolver). Current A6 phase: **A6.P3 slice 4 (or A6.P4?) —
+investigate #98 BSP step-physics at cellar stair top.**
 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