test(p0): retail find_cell_list trace parser + cdb value-capture tooling

P0 Task 5. RetailTrace parses the [fcl] golden format (seed/pos/picked,
RetailCellPick); 4 TDD tests green. find-cell-list-capture.cdb targets
CPhysicsObj::change_cell (commit-on-diff) to capture retail's accepted
membership sequence at the doorway; README is the operator runbook
(dt offset verification + decode_retail_hex float decode). The live run
is P0's one user-gated step (Task 6 mines existing traces first).

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

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

@@ -0,0 +1,56 @@
+using System.Numerics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Conformance;
+
+/// <summary>P0 Task 5 — TDD for the retail find_cell_list trace parser.</summary>
+public class RetailTraceTests
+{
+    [Fact]
+    public void Parse_FindCellListLine_YieldsSeedPosAndPicked()
+    {
+        const string line =
+            "[fcl] seed=0xA9B40170 px=141.5000 py=7.2200 pz=92.7400 picked=0xA9B40171";
+        var rec = RetailTrace.ParseFindCellList(line);
+        Assert.NotNull(rec);
+        Assert.Equal(0xA9B40170u, rec!.SeedCellId);
+        Assert.Equal(new Vector3(141.5f, 7.22f, 92.74f), rec.Position);
+        Assert.Equal(0xA9B40171u, rec.PickedCellId);
+    }
+
+    [Fact]
+    public void Parse_NegativeCoordinatesAndLowercaseHex_Ok()
+    {
+        const string line =
+            "[fcl] seed=0xa9b40031 px=-12.5 py=0 pz=-3.25 picked=0xa9b40170";
+        var rec = RetailTrace.ParseFindCellList(line);
+        Assert.NotNull(rec);
+        Assert.Equal(0xA9B40031u, rec!.SeedCellId);
+        Assert.Equal(new Vector3(-12.5f, 0f, -3.25f), rec.Position);
+        Assert.Equal(0xA9B40170u, rec.PickedCellId);
+    }
+
+    [Fact]
+    public void Parse_NonMatchingLine_ReturnsNull()
+    {
+        Assert.Null(RetailTrace.ParseFindCellList("[BP4] find_collisions hit#10170 collide=0"));
+        Assert.Null(RetailTrace.ParseFindCellList(""));
+        Assert.Null(RetailTrace.ParseFindCellList("[fcl] seed=0xA9B40170 px=1 py=2")); // truncated
+    }
+
+    [Fact]
+    public void ParseFile_SkipsNoiseAndYieldsOnlyPicks()
+    {
+        var lines = new[]
+        {
+            "armed; walk now",
+            "[fcl] seed=0xA9B40031 px=160.0 py=10.0 pz=94.0 picked=0xA9B40170",
+            "[BP] noise",
+            "[fcl] seed=0xA9B40170 px=158.0 py=12.0 pz=95.0 picked=0xA9B40171",
+        };
+        var picks = RetailTrace.ParseAll(lines);
+        Assert.Equal(2, picks.Count);
+        Assert.Equal(0xA9B40170u, picks[0].PickedCellId);
+        Assert.Equal(0xA9B40171u, picks[1].PickedCellId);
+    }
+}