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>

src/AcDream.Core/Physics/CellArray.cs

@@ -0,0 +1,56 @@
+using System.Collections;
+using System.Collections.Generic;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Ordered, deduped cell-id collection — a faithful model of retail's CELLARRAY
+/// (<c>CELLARRAY::add_cell</c> @ <c>acclient_2013_pseudo_c.txt:701036</c>: linear
+/// dedup by cell_id, append at the END, insertion order preserved). The order is
+/// load-bearing for <c>CObjCell::find_cell_list</c>'s current-cell-first,
+/// interior-wins pick (pc:308742) — the current cell is added at index 0 and the
+/// pick iterates in order, so the current cell wins a boundary straddle and the
+/// membership does not ping-pong (the R1 flap fix). Replaces the unordered
+/// <see cref="HashSet{T}"/> the candidate build used to use.
+///
+/// <para>Implements <see cref="ICollection{T}"/> (so the candidate-building
+/// helpers can take it where they used to take <c>HashSet&lt;uint&gt;</c>) and
+/// <see cref="IReadOnlyCollection{T}"/> (so it satisfies the
+/// <c>out IReadOnlyCollection&lt;uint&gt;</c> on <c>FindCellSet</c> and the
+/// <c>CheckOtherCells</c> / diagnostics consumers). Enumeration is always
+/// insertion order.</para>
+/// </summary>
+public sealed class CellArray : ICollection<uint>, IReadOnlyCollection<uint>
+{
+    private readonly List<uint> _order = new();
+    private readonly HashSet<uint> _seen = new();
+
+    public int Count => _order.Count;
+    public bool IsReadOnly => false;
+
+    /// <summary>Ordered cell ids; index 0 is the cell added first (the current cell).</summary>
+    public IReadOnlyList<uint> OrderedIds => _order;
+
+    /// <summary>Append <paramref name="id"/> iff not already present (retail add_cell dedup).</summary>
+    public void Add(uint id)
+    {
+        if (_seen.Add(id))
+            _order.Add(id);
+    }
+
+    public bool Contains(uint id) => _seen.Contains(id);
+
+    public void Clear() { _order.Clear(); _seen.Clear(); }
+
+    public bool Remove(uint id)
+    {
+        if (!_seen.Remove(id)) return false;
+        _order.Remove(id);
+        return true;
+    }
+
+    public void CopyTo(uint[] array, int arrayIndex) => _order.CopyTo(array, arrayIndex);
+
+    public IEnumerator<uint> GetEnumerator() => _order.GetEnumerator();
+    IEnumerator IEnumerable.GetEnumerator() => _order.GetEnumerator();
+}