Frame: add adaptive mesh refinement

tests/test_schwarzschild.c

@@ -1,4 +1,5 @@
 #include "geodesic.h"
+#include "frame.h"
 
 #include <math.h>
 #include <stdio.h>
@@ -56,6 +57,25 @@ int main(void) {
             central.status, inside_shadow.status, outside_shadow.status);
     goto done;
   }
+  /* A coarse field covering the shadow must genuinely refine: its initial
+   * capture/escape-discontinuous triangles are a separate trigger from the
+   * smooth direction-error criterion. */
+  FrameLensMesh mesh = {0};
+  const RefinementConfig refinement = {.max_level = 1,
+                                       .angle_absolute_rad = 1e-5,
+                                       .angle_relative = 1e-5,
+                                       .min_edge_pixels = 1.0,
+                                       .min_area_pixels2 = 1.0};
+  if (frame_lens_mesh_build_coarse(&mesh, 48, 48, 24, 40.0) ||
+      frame_lens_mesh_trace(&mesh, &spacetime, &observer, &trace) ||
+      frame_lens_mesh_refine(&mesh, &spacetime, &observer, &trace,
+                             &refinement) ||
+      mesh.vertex_count <= 9 || mesh.triangle_count <= 8) {
+    fputs("Schwarzschild adaptive-refinement regression failed\n", stderr);
+    frame_lens_mesh_destroy(&mesh);
+    goto done;
+  }
+  frame_lens_mesh_destroy(&mesh);
   result = 0;
 done:
   spacetime_destroy(&spacetime);