phase(N.5b) Task 2: TerrainSlotAllocator + tests

Pure-CPU slot allocator for the terrain modern dispatcher's global
VBO/EBO. FIFO free-list + monotonic counter, mirroring WB's
TerrainRenderManager pattern. Caller (TerrainModernRenderer) handles
GPU buffer growth when Allocate sets needsGrow=true.

8 unit tests cover: fresh-allocator returns slot 0, sequential
allocs, free+alloc reuse, FIFO ordering, needsGrow signaling on
capacity overflow, GrowTo, LoadedCount tracking, and double-free
detection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/Terrain/TerrainSlotAllocatorTests.cs

@@ -0,0 +1,88 @@
+using AcDream.Core.Terrain;
+using Xunit;
+
+namespace AcDream.Core.Tests.Terrain;
+
+public class TerrainSlotAllocatorTests
+{
+    [Fact]
+    public void Allocate_FromFreshAllocator_ReturnsZero()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 8);
+        Assert.Equal(0, alloc.Allocate(out _));
+    }
+
+    [Fact]
+    public void Allocate_TwoTimes_ReturnsZeroThenOne()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 8);
+        Assert.Equal(0, alloc.Allocate(out _));
+        Assert.Equal(1, alloc.Allocate(out _));
+    }
+
+    [Fact]
+    public void FreeThenAllocate_ReusesFreedSlot()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 8);
+        var s0 = alloc.Allocate(out _);
+        var s1 = alloc.Allocate(out _);
+        alloc.Free(s0);
+        Assert.Equal(s0, alloc.Allocate(out _));
+    }
+
+    [Fact]
+    public void FreeOrderedFreshAllocs_ReturnsInFifoOrder()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 8);
+        var s0 = alloc.Allocate(out _);
+        var s1 = alloc.Allocate(out _);
+        var s2 = alloc.Allocate(out _);
+        alloc.Free(s0);
+        alloc.Free(s2);
+        Assert.Equal(s0, alloc.Allocate(out _));
+        Assert.Equal(s2, alloc.Allocate(out _));
+    }
+
+    [Fact]
+    public void Allocate_BeyondInitialCapacity_SignalsNeedsGrow()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 2);
+        alloc.Allocate(out var grow0);
+        alloc.Allocate(out var grow1);
+        alloc.Allocate(out var grow2);
+        Assert.False(grow0);
+        Assert.False(grow1);
+        Assert.True(grow2);
+    }
+
+    [Fact]
+    public void GrowTo_DoublesCapacityCorrectly()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 4);
+        alloc.GrowTo(8);
+        Assert.Equal(8, alloc.Capacity);
+        alloc.GrowTo(64);
+        Assert.Equal(64, alloc.Capacity);
+    }
+
+    [Fact]
+    public void LoadedCount_TracksAllocAndFree()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 8);
+        Assert.Equal(0, alloc.LoadedCount);
+        var s0 = alloc.Allocate(out _);
+        var s1 = alloc.Allocate(out _);
+        Assert.Equal(2, alloc.LoadedCount);
+        alloc.Free(s0);
+        Assert.Equal(1, alloc.LoadedCount);
+    }
+
+    [Fact]
+    public void Free_TwiceForSameSlot_Throws()
+    {
+        var alloc = new TerrainSlotAllocator(initialCapacity: 8);
+        var s0 = alloc.Allocate(out _);
+        alloc.Free(s0);
+        Assert.Throws<InvalidOperationException>(() => alloc.Free(s0));
+    }
+}