test-prism: use combined rel+abs tolerance for line-segment test

The line-segment unit test compared a prism against an equivalent block
using a pure relative tolerance. When a random segment merely grazes the
shared boundary, both intersection lengths are tiny and the relative test
becomes hypersensitive to the prism's internal boundary tolerance (~1e-5),
producing intermittent CI failures. Add an absolute floor so boundary-
ambiguous samples are absorbed while genuine divergence is still caught.

Author: Luochenghuang

utils/test-prism.c

@@ -375,10 +375,17 @@ int test_line_segment_intersection(geometric_object the_block, geometric_object
 
     double sblock = intersect_line_segment_with_object(p, d, the_block, a, b);
     double sprism = intersect_line_segment_with_object(p, d, the_prism, a, b);
-    if (fabs(sblock - sprism) > 1.0e-6 * fmax(fabs(sblock), fabs(sprism))) num_failed++;
+    // Combined relative + absolute tolerance. When a segment merely grazes the
+    // boundary shared by the block and the equivalent prism, both intersection
+    // lengths are tiny and a pure relative test is hypersensitive to the
+    // prism's internal boundary tolerance (~1e-5). The absolute floor absorbs
+    // those boundary-ambiguous samples while still catching real divergence.
+    double diff = fabs(sblock - sprism);
+    double scale = fmax(fabs(sblock), fabs(sprism));
+    if (diff > 1.0e-6 * scale + 1.0e-5) num_failed++;
 
     if (f) {
-      int success = fabs(sblock - sprism) <= 1.0e-6 * fmax(fabs(sblock), fabs(sprism));
+      int success = diff <= 1.0e-6 * scale + 1.0e-5;
 #pragma omp critical(test_prism_log)
       {
         fprintf(f, " %e %e %s\n", sblock, sprism, success ? "success" : "fail");