Stable radix sprite sort and render hot-path optimizations

SpriteBatcher now sorts with a stable LSD radix sort: equal-key sprites
keep submission order across frames (no flicker) and passes over digits
identical in all keys are skipped, making the common single-layer case
nearly free. Hot paths avoid per-sprite trig and square roots: SinCos is
skipped for unrotated sprites and the culling radius comes from the
region's precomputed diagonal.

Stress scene (100k entities, ~61k on screen, Release): 103 -> 124 FPS.
Sample now runs with VSync off to show real frame rates.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/MrGameEng.Graphics.Tests/SpriteBatcherTests.cs

@@ -39,6 +39,66 @@ public class SpriteBatcherTests
         Assert.Equal(99, order[0]); // последний сабмит имеет наименьший ключ
     }
 
+    [Fact]
+    public void Sort_IsStable_EqualKeysKeepSubmissionOrder()
+    {
+        var batcher = new SpriteBatcher();
+        var key = SpriteSortKey.Make(1, 5f, 7);
+
+        for (byte i = 0; i < 50; i++)
+        {
+            batcher.Submit(Instance(i), key); // одинаковый ключ у всех
+        }
+
+        var order = batcher.Sort();
+
+        for (var i = 0; i < 50; i++)
+        {
+            Assert.Equal(i, order[i]);
+        }
+    }
+
+    [Fact]
+    public void Sort_IsStable_WithinMixedKeys()
+    {
+        var batcher = new SpriteBatcher();
+        var keyA = SpriteSortKey.Make(0, 0f, 1);
+        var keyB = SpriteSortKey.Make(2, 3f, 9);
+
+        // Чередуем два ключа: внутри каждой группы порядок сабмита должен сохраниться.
+        for (byte i = 0; i < 20; i++)
+        {
+            batcher.Submit(Instance(i), i % 2 == 0 ? keyA : keyB);
+        }
+
+        var order = batcher.Sort();
+
+        var expectedA = new[] { 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 };
+        var expectedB = new[] { 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 };
+        Assert.Equal(expectedA, order[..10].ToArray());
+        Assert.Equal(expectedB, order[10..].ToArray());
+    }
+
+    [Fact]
+    public void Sort_LargeRandomSet_FullyOrdered()
+    {
+        var batcher = new SpriteBatcher(initialCapacity: 16);
+        var random = new Random(123);
+        var keys = new ulong[5000];
+        for (var i = 0; i < keys.Length; i++)
+        {
+            keys[i] = (ulong)random.NextInt64();
+            batcher.Submit(Instance(0), keys[i]);
+        }
+
+        var order = batcher.Sort();
+
+        for (var i = 1; i < order.Length; i++)
+        {
+            Assert.True(keys[order[i - 1]] <= keys[order[i]]);
+        }
+    }
+
     [Fact]
     public void Clear_ResetsCount_KeepsWorking()
     {