Feat: Add post-tone-map mesh diagnostics with RGBA overlays

Color antialiased half-edges by ray outcome with configurable Catppuccin colors and default opacity 0.5.

Rasterize premultiplied RGBA8 overlays on the producer and composite in place after writing the clean image. Keep single-frame, movie, and replay output consistent.

Add overlay, CLI, and queue ownership regressions and document the final output architecture.

README.zh-CN.md

@@ -119,6 +119,7 @@ mkdir -p output/imgs
 ### 示例：叠加网格的合成测试星表
 
 这个示例使用 `assets/sky_grid_5deg.csv` 检查 Schwarzschild 时空中的引力透镜效果与自适应网格细分。主输出是不带网格的成品图；`--draw-mesh` 会额外写出最终的像平面三角网格，因此同一次命令会同时生成 `output/imgs/schwarzschild_test_grid.png`（无网格）和 `output/imgs/schwarzschild_test_grid_mesh.png`（网格叠加）。
+网格在 tone mapping 后以抗锯齿半边叠加。默认采用 Catppuccin Mocha：逃逸为灰色、暗终态为紫色、预算耗尽未决为黄色、真实失败为红色，未追踪为蓝色。颜色与透明度配置详见 `usage.md`。
 
 ```sh
 mkdir -p output/imgs