test(p0): find_cell_list golden conformance — unambiguous interior picks

P0 Task 4. FindCellList resolves a sphere deep inside room 0171 -> 0171,
deep inside vestibule 0170 -> 0170, and re-picks 0170 from a stale 0171
seed (membership re-picks by containment, not the seed). Retail-faithful
by construction (candidate cells loaded from the real dats). The subtle
doorway-threshold pick is the trace-backed golden (Task 6).

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

tests/AcDream.Core.Tests/Conformance/FindCellListConformanceTests.cs

@@ -0,0 +1,87 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+using DatReaderWriter;
+using DatReaderWriter.Options;
+using Xunit;
+
+namespace AcDream.Core.Tests.Conformance;
+
+/// <summary>
+/// P0 Task 4 — golden conformance for the membership pick
+/// <see cref="CellTransit.FindCellList"/> (retail CObjCell::find_cell_list
+/// @ 0x52b4e0 pc:308742). The unambiguous cases are retail-faithful by
+/// construction: the candidate cells are loaded from the same dats retail
+/// loads, and a sphere clearly inside one room can only resolve to that
+/// room. The subtle doorway-threshold pick (the 0031↔0170↔0171 ping-pong)
+/// is the trace-backed golden in FindCellListConformanceTests (Task 6).
+/// </summary>
+public class FindCellListConformanceTests
+{
+    private const float FootRadius = 0.4f; // player foot-sphere radius
+
+    /// <summary>Cache the whole threshold building (016F..0175) so portal expansion is faithful.</summary>
+    private static PhysicsDataCache LoadThresholdBuilding(DatCollection dats)
+    {
+        var cache = new PhysicsDataCache();
+        for (uint low = 0x016Fu; low <= 0x0175u; low++)
+            ConformanceDats.LoadEnvCell(dats, cache, ConformanceDats.HoltburgLandblock | low);
+        return cache;
+    }
+
+    [Fact]
+    public void FindCellList_DeepInsideRoom0171_Returns0171()
+    {
+        var datDir = ConformanceDats.ResolveDatDir();
+        if (datDir is null) return;
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var cache = LoadThresholdBuilding(dats);
+
+        var world = Vector3.Transform(
+            CottageDoorwayCharacterizationTests.Interior0171Local,
+            cache.GetCellStruct(CottageDoorwayCharacterizationTests.Room0171)!.WorldTransform);
+
+        uint picked = CellTransit.FindCellList(cache, world, FootRadius,
+            CottageDoorwayCharacterizationTests.Room0171);
+        Assert.Equal(CottageDoorwayCharacterizationTests.Room0171, picked);
+    }
+
+    [Fact]
+    public void FindCellList_DeepInsideVestibule0170_Returns0170()
+    {
+        var datDir = ConformanceDats.ResolveDatDir();
+        if (datDir is null) return;
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var cache = LoadThresholdBuilding(dats);
+
+        var world = Vector3.Transform(
+            CottageDoorwayCharacterizationTests.Interior0170Local,
+            cache.GetCellStruct(CottageDoorwayCharacterizationTests.Vestibule0170)!.WorldTransform);
+
+        uint picked = CellTransit.FindCellList(cache, world, FootRadius,
+            CottageDoorwayCharacterizationTests.Vestibule0170);
+        Assert.Equal(CottageDoorwayCharacterizationTests.Vestibule0170, picked);
+    }
+
+    /// <summary>
+    /// Seeding from the WRONG current cell (room 0171) while standing deep inside
+    /// the vestibule (0170) must still resolve to 0170 — find_cell_list re-picks by
+    /// containment, it does not trust the stale seed. This is the membership-stability
+    /// property P1 must preserve.
+    /// </summary>
+    [Fact]
+    public void FindCellList_InVestibule_SeededFromRoom_Returns0170()
+    {
+        var datDir = ConformanceDats.ResolveDatDir();
+        if (datDir is null) return;
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var cache = LoadThresholdBuilding(dats);
+
+        var world = Vector3.Transform(
+            CottageDoorwayCharacterizationTests.Interior0170Local,
+            cache.GetCellStruct(CottageDoorwayCharacterizationTests.Vestibule0170)!.WorldTransform);
+
+        uint picked = CellTransit.FindCellList(cache, world, FootRadius,
+            CottageDoorwayCharacterizationTests.Room0171); // stale/wrong seed
+        Assert.Equal(CottageDoorwayCharacterizationTests.Vestibule0170, picked);
+    }
+}