feat(physics): Stage 1 — CellArray ordered/deduped cell collection (retail CELLARRAY)

Ports retail CELLARRAY::add_cell (acclient_2013_pseudo_c.txt:701036): ordered list,
dedup by cell_id, append at end. The order is load-bearing for the verbatim
find_cell_list current-cell-first interior-wins pick (next commits) that fixes the
R1 cottage membership flap. Implements ICollection<uint> (helper-facing) +
IReadOnlyCollection<uint> (consumer-facing). 5 unit tests.

Also lands the membership-port pseudocode (workflow step 3) + the Stage-1 plan.

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

tests/AcDream.Core.Tests/Physics/CellArrayTests.cs

@@ -0,0 +1,59 @@
+using System.Collections.Generic;
+using System.Linq;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class CellArrayTests
+{
+    [Fact]
+    public void Add_PreservesInsertionOrder()
+    {
+        var a = new CellArray();
+        a.Add(0xA9B40170u);
+        a.Add(0xA9B40031u);
+        a.Add(0xA9B40171u);
+        Assert.Equal(new[] { 0xA9B40170u, 0xA9B40031u, 0xA9B40171u }, a.OrderedIds.ToArray());
+    }
+
+    [Fact]
+    public void Add_DedupsById_KeepingFirstPosition()
+    {
+        var a = new CellArray();
+        a.Add(0xA9B40170u);
+        a.Add(0xA9B40171u);
+        a.Add(0xA9B40170u);            // duplicate of index 0 — no-op (retail add_cell)
+        Assert.Equal(2, a.Count);
+        Assert.Equal(new[] { 0xA9B40170u, 0xA9B40171u }, a.OrderedIds.ToArray());
+    }
+
+    [Fact]
+    public void Contains_TracksMembership()
+    {
+        var a = new CellArray();
+        a.Add(0xA9B40170u);
+        Assert.True(a.Contains(0xA9B40170u));
+        Assert.False(a.Contains(0xA9B40171u));
+    }
+
+    [Fact]
+    public void EnumeratesInInsertionOrder_AsICollection()
+    {
+        var a = new CellArray();
+        a.Add(3u); a.Add(1u); a.Add(2u);
+        ICollection<uint> c = a;               // helper-facing interface
+        Assert.Equal(new[] { 3u, 1u, 2u }, c.ToArray());
+    }
+
+    [Fact]
+    public void IsReadOnlyCollection_ForConsumers()
+    {
+        var a = new CellArray();
+        a.Add(7u); a.Add(7u);
+        IReadOnlyCollection<uint> ro = a;      // consumer-facing interface (FindCellSet out)
+        int count = ro.Count;                  // local avoids xUnit2013 (still exercises .Count)
+        Assert.Equal(1, count);
+        Assert.Equal(new[] { 7u }, ro.ToArray());
+    }
+}