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>

samples/MrGameEng.Sample/Program.cs

@@ -9,6 +9,7 @@ using var host = new GameHost(
         Width = 1280,
         Height = 720,
         ClearColor = new Color(24, 26, 32),
+        VSync = false, // техдемо: показываем реальный FPS, не ограниченный частотой монитора
     },
     new MainScene());