Shape caches: behavioural tests (CenterOfMass and AllCoordinates stale after an edit; Resize fill kept)

Builds; ShapeCacheTests.CenterOfMassFollowsACellCleared and ResizeRecountsEveryCoordinateRead fail at this commit by design (fixed in the next).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Aetheria.Shared.Tests/ShapeCacheTests.cs

@@ -0,0 +1,53 @@
+using System.Linq;
+using CultMath;
+using Xunit;
+using static CultMath.math;
+
+// Shape's derived reads (Coordinates, AllCoordinates, CenterOfMass) are caches of Cells and follow every edit.
+public sealed class ShapeCacheTests
+{
+    [Fact]
+    public void CenterOfMassFollowsACellCleared()
+    {
+        var shape = new Shape(2, 1);
+        shape[int2(0, 0)] = true;
+        shape[int2(1, 0)] = true;
+        Assert.Equal(0.5f, shape.CenterOfMass.x);
+
+        shape[int2(0, 0)] = false;
+
+        Assert.Equal(1f, shape.CenterOfMass.x);
+        Assert.Equal(0f, shape.CenterOfMass.y);
+    }
+
+    [Fact]
+    public void ResizeRecountsEveryCoordinateRead()
+    {
+        var shape = new Shape(2, 1);
+        shape[int2(0, 0)] = true;
+        shape[int2(1, 0)] = true;
+        Assert.Equal(2, shape.AllCoordinates.Length);
+        Assert.Equal(2, shape.Coordinates.Length);
+        var before = shape.CenterOfMass;
+
+        shape.Resize(3, 2);
+
+        Assert.Equal(6, shape.AllCoordinates.Length);
+        Assert.Equal(shape.Cells.Cast<bool>().Count(c => c), shape.Coordinates.Length);
+        Assert.NotEqual(before, shape.CenterOfMass);
+    }
+
+    [Fact]
+    public void ResizeKeepsExistingCellsAndFillsNewOnes()
+    {
+        // The hull drawer's width grow relies on the fill (ships-studio-footprint r1 keeps it).
+        var shape = new Shape(2, 1);
+        shape[int2(0, 0)] = true;
+
+        shape.Resize(3, 1);
+
+        Assert.True(shape[int2(0, 0)]);
+        Assert.False(shape[int2(1, 0)]);
+        Assert.True(shape[int2(2, 0)]);
+    }
+}