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.

tests/test_frame.c

@@ -1,5 +1,6 @@
 #include "frame.h"
 #include "lens_map.h"
+#include "mesh_overlay.h"
 #include "optics.h"
 
 #include <math.h>
@@ -1086,10 +1087,27 @@ int main(void) {
     if (!fast_ok)
       goto done;
   }
-  frame_draw_mesh(&mesh, hdr, width, height, 0.5, 0.5);
-  if (hdr[3 * (10 * width + 20)] != 0.25) {
-    fputs("mesh diagnostic overlay regression failed\n", stderr);
-    goto done;
+  /* Diagnostic overlay draws the finalized mesh as an sRGB8 edge map: the
+   * vertical coarse edge crossing (20, 10) must be painted, while an interior
+   * pixel away from every edge must stay at the background value. */
+  {
+    MeshOverlayLines overlay_lines = {0};
+    const MeshOverlaySettings overlay_settings = mesh_overlay_default_settings();
+    unsigned char *overlay_rgb = calloc((size_t)width * height * 3, 1);
+    const int overlay_ok =
+        overlay_rgb != NULL &&
+        mesh_overlay_prepare(&mesh, &overlay_lines) == 0 &&
+        overlay_lines.count != 0 &&
+        mesh_overlay_draw_rgb8(&overlay_lines, overlay_rgb, width, height,
+                               &overlay_settings) == 0 &&
+        overlay_rgb[3 * (10 * width + 20)] != 0 &&
+        overlay_rgb[3 * (12 * width + 5)] == 0;
+    mesh_overlay_lines_destroy(&overlay_lines);
+    free(overlay_rgb);
+    if (!overlay_ok) {
+      fputs("mesh diagnostic overlay regression failed\n", stderr);
+      goto done;
+    }
   }
   /* A fixed absolute edge tolerance used to make tiny source triangles claim
    * sources far outside their field. */