docs: add verify-first kickoff prompt to the render-residuals handoff (treat the diagnosis as a suspect statement)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/research/2026-06-07-cutover-flip-render-residuals-diagnosis-handoff.md

@@ -123,3 +123,76 @@ read. This answers whether F2 is "build back faces" or "restore a pre-flip draw"
 - Memory: `project_indoor_flap_rootcause` (update with this corrected diagnosis),
   `reference_render_pipeline_state`, `feedback_render_downstream_of_membership` (the oscillation IS a
   membership/flood-stability bug, per that note).
+
+---
+
+## 7. Next-session kickoff prompt — VERIFY-FIRST (the diagnosis above is a SUSPECT'S statement)
+
+This session reached the §3 diagnosis only AFTER three wrong guesses, so the next session must verify it
+cold before building any fix. Paste this to start:
+
+```
+Continue acdream M1.5 render unification. Branch claude/thirsty-goldberg-51bb9b, HEAD 774cb22.
+PowerShell on Windows; build before launch; live ACE 127.0.0.1:9000, testaccount/testpassword, char
++Acdream (spawns at the Holtburg/Arcanum cottage, landblock 0xA9B4).
+
+The render-unification CUTOVER FLIP is committed. It is CLAIMED to have killed the two-branch render
+"flap" but left two residuals — see-through building walls and oscillation — and a root-cause diagnosis
+was reached. That diagnosis was only reached AFTER THREE WRONG GUESSES in the same session, so DO NOT
+trust it. Your FIRST job is to verify it cold, with fresh primary evidence, and you are explicitly
+licensed to REFUTE it.
+
+READ (as a suspect's statement, NOT as truth):
+- docs/research/2026-06-07-cutover-flip-render-residuals-diagnosis-handoff.md  (claimed diagnosis +
+  do-not-retry list + the probe numbers it rests on)
+- memory project_indoor_flap_rootcause (corrected diagnosis claim)
+
+=== TASK 1 — UNBIASED VERIFICATION (complete fully BEFORE proposing any fix) ===
+Do not anchor on the handoff's conclusions — re-derive each from independent evidence. For each claim,
+report CONFIRMED / REFUTED / CORRECTED with the evidence. If ANY load-bearing claim is refuted, the
+diagnosis is wrong: STOP, re-diagnose, do not build a fix on it. Prefer dispatching the verification to a
+fresh subagent that has NOT seen the conclusions, to avoid confirmation bias.
+
+1.1 "The branch-toggle flap is gone (one render path)." Launch (ACDREAM_PROBE_FLAP=1); walk
+    indoor<->outdoor and pan the camera at a doorway; RLE the [render-sig] `branch` field. Expected if
+    true: zero `branch=OutdoorRoot` after spawn. Refute if OutdoorRoot reappears.
+
+1.2 "Oscillation == outdoor-node flood membership instability." Add a probe logging
+    pvFrame.OrderedVisibleCells.Count per outdoor-root frame WHILE STANDING PERFECTLY STILL at the
+    cottage. Swings frame-to-frame (e.g. 1<->13) -> unstable (confirms). Constant while the user still
+    sees oscillation -> DIFFERENT cause (refute + re-diagnose). Correlate the swings with what visibly
+    flickers.
+
+1.3 "See-through == single-sided EnvCell walls." Dump the actual sides_type distribution of a REAL
+    Holtburg building cell's wall polygons (Environment dat -> CellStruct, focused test/tool over the
+    cottage cell). Confirm walls are predominantly single-sided AND that PrepareCellStructMeshData
+    (ObjectMeshManager ~1299-1310) builds a back face only for SidesType None/Clockwise (not
+    CounterClockwise). FALSIFIABLE cross-check: temporarily force the EnvCell shell pass to CullMode.None
+    (double-sided) and confirm THAT alone makes the walls solid from outside; revert after.
+
+1.4 "The building WALLS are the EnvCell shells; the ModelId 'shell' is only a partial frame." Re-add the
+    [bshell] probe (total/withMesh/inOutdoorPartition/envCellsFlooded). Independently inspect what the
+    building ModelId Setup geometry IS (poly count, bbox) vs the EnvCell shell. Reproduce or refute the
+    skip-all-interiors experiment (building went fully see-through).
+
+1.5 (OPEN, decides the fix shape) "Pre-flip buildings were solid from outside — what drew the walls?"
+    Check out the pre-flip commit (parent of 445e861), launch, confirm buildings solid from outside,
+    trace what drew the solid walls (old outdoor `else` block GameWindow.cs ~7557-7663 /
+    DrawPortal+BuildFromExterior / a global EnvCell-shell render). Decides whether F2 is "build missing
+    back faces" or "restore a pre-flip draw the flip replaced".
+
+DO-NOT-RETRY (proven failures last session): the slot-0 skip (made oscillation worse); skipping all
+interior shells / gating DrawEnvCellShells off for the outdoor root (building fully see-through — already
+ran); any render-side debounce/grace (forbidden, no-workarounds).
+
+=== TASK 2 — only AFTER Task 1 confirms or corrects the diagnosis ===
+Implement F1 (stabilise flood membership — e.g. ground building membership in the cell stab_list/PVS
+instead of the per-frame portal-side test) and F2 (the verified wall-sidedness fix). TDD where possible;
+each lands behind a USER VISUAL GATE at the cottage. Do not delete the dead DrawPortal/BuildFromExterior/
+outdoor-else paths until the residuals are visually confirmed fixed.
+```
+
+**Why verify-first:** the fastest single decisive test is the §1.3 falsifiable cross-check (force
+`CullMode.None`; if walls go solid from outside, the single-sided-wall hypothesis is confirmed and F2 is
+"build back faces"). Run the verification under a fresh subagent so it can't rubber-stamp these
+conclusions.