feat(lighting): SelectForCell (all dynamic lights per EnvCell) + #176 handoff — residual is a runtime z-fight

cdb trace of LIVE retail (tools/cdb/issue176-floor-light.cdb, binary<->PDB MATCH) PROVED retail
applies ALL its dynamic lights — 4 intensity-100 magenta portal lights (d3dIdx 3-6, falloff 6) +
the viewer fill (d3dIdx 1, 2.25) — as D3D hardware lights to EVERY Facility Hub cell, every frame,
stable. So the faceted purple wedges on the floor are retail-FAITHFUL. acdream did a per-cell
SelectForObject sphere-overlap 8-cap for cells, so the portal set could differ/flip per cell.

- LightManager.SelectForCell (retail minimize_envcell_lighting 0x0054c170): ALL dynamic lights
  applied unconditionally (shader range cutoff zeroes non-reaching = D3D hardware range), then
  nearest static torches fill remaining slots. Wired into EnvCellRenderer.GetCellLightSet.
  Objects keep SelectForObject (minimize_object_lighting). Pins:
  SelectForCell_AppliesAllDynamicLights_EvenOutOfReach + _SameDynamicSet_ForCellsFarApart_NoFlap.
- Apparatus: [light-detail] gains owner/cell/dyn (pinned the culprit = 2 portal weenies
  0x000F4247/48 in 0x8A020118/19, intensity=100 magenta); CellVertexNormals_SmoothOrFaceted_Dump
  (corridor floor uses SMOOTH per-vertex dat normals, not flat); tools/cdb/issue176-floor-light.cdb.

#176 RESIDUAL is NOT this fix. It's a RUNTIME draw z-fight in the seam floor. Eliminated (evidence):
NOT lighting (per-light cap + this both no-change), NOT membership (render cell 0x8A020164 stable
100% of 188k frames / 526 angles, res=None), NOT dat geometry (coplanar sweep empty at z=-6 floor
incl. cell 0164). NEXT = RenderDoc pixel-history. Full handoff + DO-NOT-RETRY:
docs/research/2026-07-06-176-seam-floor-zfight-handoff.md. Suites green: Core 2599 + 2 skip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/AcDream.Core.Tests/Lighting/LightManagerTests.cs

@@ -17,6 +17,17 @@ public sealed class LightManagerTests
             CellId = cellId,
         };
 
+    private static LightSource MakeDynamic(Vector3 pos, float range, uint cellId = 0)
+        => new LightSource
+        {
+            Kind = LightKind.Point,
+            WorldPosition = pos,
+            Range = range,
+            IsLit = true,
+            IsDynamic = true,
+            CellId = cellId,
+        };
+
     [Fact]
     public void Register_Unregister_TracksList()
     {
@@ -307,6 +318,58 @@ public sealed class LightManagerTests
         Assert.Equal(a[0], b[0]);
     }
 
+    // ── SelectForCell — retail minimize_envcell_lighting (all dynamics on every cell) ──
+
+    [Fact]
+    public void SelectForCell_AppliesAllDynamicLights_EvenOutOfReach()
+    {
+        // Retail enables the WHOLE dynamic subset for every cell (cdb-verified: the same
+        // portal lights on every Facility Hub cell) — including ones that don't reach it,
+        // since the shader's range cutoff zeroes those. Static lights still cull by reach.
+        var snapshot = new[]
+        {
+            MakePoint(new Vector3(1, 0, 0), range: 5f),      // 0: static, reaches
+            MakeDynamic(new Vector3(100, 0, 0), range: 5f),  // 1: dynamic, FAR (out of reach)
+            MakeDynamic(new Vector3(2, 0, 0), range: 5f),    // 2: dynamic, near
+            MakePoint(new Vector3(50, 0, 0), range: 5f),     // 3: static, far (out of reach)
+        };
+        Span<int> sel = stackalloc int[LightManager.MaxLightsPerObject];
+        int n = LightManager.SelectForCell(snapshot, Vector3.Zero, radius: 1f, sel);
+
+        bool d1 = false, d2 = false, s0 = false, s3 = false;
+        for (int i = 0; i < n; i++)
+        {
+            if (sel[i] == 1) d1 = true;
+            if (sel[i] == 2) d2 = true;
+            if (sel[i] == 0) s0 = true;
+            if (sel[i] == 3) s3 = true;
+        }
+        Assert.True(d1, "the FAR dynamic light must still be applied — retail enables all dynamics");
+        Assert.True(d2, "the near dynamic light is applied");
+        Assert.True(s0, "the near static light reaches the cell → selected");
+        Assert.False(s3, "the far static light doesn't reach → not selected");
+    }
+
+    [Fact]
+    public void SelectForCell_SameDynamicSet_ForCellsFarApart_NoFlap()
+    {
+        // The stability retail has and we lacked: two cells far apart get the SAME dynamic
+        // set. A per-cell sphere-overlap cull of dynamics (the old SelectForObject path) let
+        // that set differ/flip as the flood shifted → the floor lighting FLAPPED (#176).
+        var snapshot = new[]
+        {
+            MakeDynamic(new Vector3(0, 0, 0), range: 5f),
+            MakeDynamic(new Vector3(100, 0, 0), range: 5f),
+        };
+        Span<int> a = stackalloc int[8];
+        Span<int> b = stackalloc int[8];
+        int na = LightManager.SelectForCell(snapshot, new Vector3(0, 0, 0), 1f, a);
+        int nb = LightManager.SelectForCell(snapshot, new Vector3(500, 0, 0), 1f, b);
+
+        Assert.Equal(2, na);   // both dynamics on the near cell
+        Assert.Equal(2, nb);   // both dynamics on the far cell too — identical, no flap
+    }
+
     /// <summary>
     /// #176/#177 (2026-07-06) — the end-state pin, via the SHIPPED fix (visible-cell
     /// scoping, not "uncap"). Before: <c>BuildPointLightSnapshot</c> kept only the