diag #181 (refined): 0x0181's region is a zero-area sliver; the amplifier is per-drawn-cell state shifting with the cell list, not light-pool scoping

Diagnostic_FlappingCellViewRegion_SliverOrLarge: at the flap pose the admitted region is one degenerate triangle (ndcArea ~0), and ClipPlaneSet.From handles degenerates correctly (area<1e-7 -> Empty) - the cell's own gated geometry costs ~zero pixels either way. The a7 pseudocode CORRECTION-2 re-read kills the flood-scoped-lights framing (the pool is already resident+player-anchored since d8984e87). Remaining suspects: per-cell light-set SSBO slot assignment (SelectForCell) or the seal/punch assembly keyed to the drawn-cell list. Next instrument: parked ACDREAM_PROBE_SEAMDRAW=1 run, diff the washed cell's [seam-blk] applied-set lines between vis=31 and vis=32 frames. ISSUES #181 + render digest updated.

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

docs/ISSUES.md

@@ -91,12 +91,23 @@ may never have been disabled.
    independent — a sliver cell flapping costs retail a few pixels; it costs us whole lit
    regions (the washed-region strobe).
 
-**The fix:** replace camera-flood light scoping with retail's registration-time light
+**REFINEMENT (same night, after the a7-pseudocode CORRECTION-2 re-read):** the pool is
-reach sets (per-light cell-set walk from the light's own cell through the portal graph;
+ALREADY resident-scoped + player-anchored (`d8984e87` deleted the flood-scoped slice-1),
-decomp pseudocode already on disk:
+so "flood-scoped lights" is NOT the amplifier as first framed. Measured: 0x0181's
-`docs/research/2026-07-06-a7-per-cell-lighting-pseudocode.md`). The wall-press wobble
+admitted view region at the flap pose is ONE zero-NDC-area sliver triangle
-and the binary knife-edge admission then match retail's visible behavior (sub-pixel
+(`Diagnostic_FlappingCellViewRegion_SliverOrLarge`), and `ClipPlaneSet.From` handles
-consequences). No hysteresis/damping band-aids on the flood.
+degenerates correctly (area < 1e-7 → Empty; sliver planes otherwise) — so the cell's own
+gated geometry costs ~zero pixels either way. **The amplifier is downstream of ADMISSION
+but not the slice gate: 0x0181 joining/leaving changes the DRAWN-CELL LIST, and some
+per-drawn-cell state keyed by list position/rebuild — prime suspects: the per-cell
+light-set SSBO slots (`SelectForCell`, d8984e87) shifting so a DIFFERENT cell reads the
+wrong 8-light set on minority frames, or the seal/punch assembly — flips a whole cell's
+lighting.** That is cell-sized, matches the captures (washed region with hard
+boundaries), and is directly instrumentable: run parked with `ACDREAM_PROBE_SEAMDRAW=1`
+(+FLAP) and diff the `[seam-blk]` applied-set lines of the WASHED cell between vis=31
+and vis=32 frames — if its applied set (or slot index) changes with 0x0181's admission,
+the indexing/rebuild site is the defect. NO band-aids on the flood admission itself
+(the sliver flap is retail-class).
 
 **Acceptance:** parked pressed camera in the washed spot → lit regions steady (the
 `[flap]` vis 31↔32 flap may legitimately persist at sub-pixel visual cost); frame-pair

tests/AcDream.App.Tests/Rendering/Issue181VisFlapReplayTests.cs

@@ -55,6 +55,65 @@ public class Issue181VisFlapReplayTests
     private static string CellSetString(IEnumerable<uint> ids)
         => string.Join(" ", ids.Select(id => $"{id & 0xFFFFu:X4}"));
 
+    [Fact]
+    public void Diagnostic_FlappingCellViewRegion_SliverOrLarge()
+    {
+        var datDir = CornerFloodReplayTests.ResolveDatDir();
+        if (datDir is null) { _out.WriteLine("SKIP: no dat dir"); return; }
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var cells = Issue120ReciprocalPingPongTests.LoadAllInteriorCells(dats, FacilityHub);
+        uint flapper = FacilityHub | 0x0181u;
+
+        // The most unstable pose from the perturbation sweep.
+        float yaw = 15f * MathF.PI / 180f, pitch = -20f * MathF.PI / 180f;
+        var gaze = new Vector3(
+            MathF.Cos(pitch) * MathF.Cos(yaw),
+            MathF.Cos(pitch) * MathF.Sin(yaw),
+            MathF.Sin(pitch));
+
+        Func<uint, LoadedCell?> lookup = id => cells.TryGetValue(id, out var c) ? c : null;
+
+        foreach (var (label, eye) in new (string, Vector3)[]
+        {
+            ("base      ", LiveEye),
+            ("+0.5mm x  ", LiveEye + new Vector3(0.0005f, 0, 0)),
+            ("-0.5mm x  ", LiveEye - new Vector3(0.0005f, 0, 0)),
+            ("+5mm x    ", LiveEye + new Vector3(0.005f, 0, 0)),
+            ("-5mm x    ", LiveEye - new Vector3(0.005f, 0, 0)),
+            ("+2cm z    ", LiveEye + new Vector3(0, 0, 0.02f)),
+            ("-2cm z    ", LiveEye - new Vector3(0, 0, 0.02f)),
+        })
+        {
+            var frame = PortalVisibilityBuilder.Build(
+                cells[LiveRoot], eye, lookup, ViewProjFor(eye, gaze),
+                buildingMembership: null,
+                drawLiftZ: PortalVisibilityBuilder.ShellDrawLiftZ);
+
+            if (!frame.CellViews.TryGetValue(flapper, out var view) || view.IsEmpty)
+            {
+                _out.WriteLine($"{label}: 0181 NOT ADMITTED");
+                continue;
+            }
+
+            // NDC shoelace area of the admitted region (screen is 4.0 NDC-units total).
+            float area = 0f; int polys = 0, verts = 0;
+            foreach (var vp in view.Polygons)
+            {
+                var v = vp.Vertices;
+                polys++; verts += v.Length;
+                float a = 0f;
+                for (int i = 0; i < v.Length; i++)
+                {
+                    var p = v[i]; var q = v[(i + 1) % v.Length];
+                    a += p.X * q.Y - q.X * p.Y;
+                }
+                area += MathF.Abs(a) * 0.5f;
+            }
+            _out.WriteLine(FormattableString.Invariant(
+                $"{label}: 0181 admitted polys={polys} verts={verts} ndcArea={area:F4} ({area / 4f * 100f:F1}% of screen)"));
+        }
+    }
+
     [Fact]
     public void Diagnostic_SubMillimeterEyePerturbation_MustNotChangeAdmission()
     {