docs(roadmap): A4 shipped + #90 cell-tracking ping-pong filed

Phase A4 (multi-cell BSP iteration) ships in three commits (e6369e2,
493c5e5, 691493e — with revert 3add110 + reapply during visual
verification that proved A4 is not the cause of the issue surfaced).
1139 + 8 baseline maintained. 10 new unit tests pass. Wires retail's
CTransition::check_other_cells (acclient_2013_pseudo_c.txt:272717-272798)
into Transition.FindEnvCollisions.

Visual verification at the Holtburg inn vestibule surfaced a separate,
pre-existing M2 blocker (filed as #90): CellId ping-pongs between
outdoor 0xA9B40022 and indoor 0xA9B40164 on every wall push-back
because the push-back exits the indoor CellBSP volume, causing the
resolver to flip back to outdoor and bypass walls on outdoor ticks.
Indoor BSP results (Collided/Adjusted/Slid all firing) prove walls ARE
detected when the player is indoor; the aggregate "walls walk through"
appearance comes from CellId classification instability, not from
collision detection.

Bug reproduces fully with A4 reverted (launch-revert2.log captured 18
cell-id flips between 0xA9B40022 ↔ 0xA9B40164, 11 inside=True
building-transit events, 61 indoor-bsp queries firing the full
result distribution). A4 is correct and tested but dormant in
practice until #90 is fixed.

Updates:
  - docs/research/2026-05-20-phase-a4-shipped-cell-pingpong-finding.md (new)
  - docs/plans/2026-04-11-roadmap.md (A4 shipped row added)
  - CLAUDE.md (Indoor walking Phase A4 paragraph + next-step pointer
    to #90 with retail oracle anchor at acclient_2013_pseudo_c.txt:308742-308783)
  - docs/ISSUES.md (#90 filed, HIGH severity, M2-blocker)
  - docs/research/2026-05-21-open-items-pickup-prompt.md (landscape
    table updated — A4 closed, #90 promoted to top blocker)

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

CLAUDE.md

@@ -801,6 +801,38 @@ Diagnostic infrastructure: `[indoor-bsp]`, `[cell-cache]`, `[cell-transit]`,
 Handoff: [`docs/research/2026-05-19-indoor-walking-phase2-shipped-handoff.md`](docs/research/2026-05-19-indoor-walking-phase2-shipped-handoff.md).
 Phase 1 handoff: [`docs/research/2026-05-19-cluster-a-shipped-handoff.md`](docs/research/2026-05-19-cluster-a-shipped-handoff.md).
 
+**Indoor walking Phase A4 — Multi-cell BSP iteration shipped 2026-05-20.**
+Three commits land the slices (with one revert/reapply during visual
+verification proving A4 wasn't the cause of the bug that surfaced):
+- `e6369e2` — `CellTransit.FindCellSet` overload exposes the candidate set
+- `493c5e5` — `Transition.CheckOtherCells` + `ApplyOtherCellResult` combine helper
+- `691493e` — wire `CheckOtherCells` into `FindEnvCollisions` (orig `967d065`, revert `3add110`, reapply)
+
+Ports retail's `CTransition::check_other_cells` at
+`acclient_2013_pseudo_c.txt:272717-272798`. After the primary cell's BSP
+returns OK, every other cell the foot-sphere overlaps is queried. Halt
+on first Collided/Adjusted/Slid; Slid clears the contact-plane fields.
+10 new unit tests; 1139 + 8 baseline maintained.
+
+**Visual verification surfaced a separate, pre-existing M2 blocker**:
+at the Holtburg inn doorway, the CellId ping-pongs between outdoor
+`0xA9B40022` and indoor vestibule `0xA9B40164` every few ticks. Indoor
+BSP DOES detect walls (Collided/Adjusted/Slid fire on push-back), but
+the push-back exits the indoor CellBSP volume → ResolveCellId
+reclassifies as outdoor → wall checks bypassed on outdoor ticks → net
+appearance "walls walk through." Bug reproduces fully with A4 reverted
+(see `launch-revert2.log`), confirming A4 is not the cause. A4 is
+correct and tested but **dormant in practice** until the ping-pong is
+fixed. Handoff:
+[`docs/research/2026-05-20-phase-a4-shipped-cell-pingpong-finding.md`](docs/research/2026-05-20-phase-a4-shipped-cell-pingpong-finding.md).
+
+**Next: cell-tracking ping-pong fix.** Retail oracle:
+`acclient_2013_pseudo_c.txt:308742-308783` (`CObjCell::find_cell_list`
+Position-variant). Look for the cell-array hysteresis / stickiness
+logic that prevents flipping CellId on a single push-back. Likely
+modifies `PhysicsEngine.ResolveCellId` to prefer the previous indoor
+classification when the sphere is close to the indoor CellBSP volume.
+
 **Next phase is Claude's choice** per work-order autonomy. Candidates:
 M2 critical path (F.2 / F.3 / F.5a / L.1c / L.1b — kill-a-drudge demo);
 or the pre-existing "next phase candidates" list below.