ShadowFinder: the arm search's dim plane as a zero-filled Plane, for the merged helpers

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D1G8gJVAy6gp1K5Dz3NE5C

image_analysis/beam_stop/ShadowFinder.cpp

@@ -958,7 +958,7 @@ std::vector<uint32_t> ShadowFinder::GetMask(size_t nthreads) const {
         }
     });
 
-    std::vector<char> dim(n_pixels, 0);
+    Plane<char> dim = filled_plane<char>(n_pixels, 0, nthreads);
     ParallelChunks(n_pixels, nthreads, [&](int lo, int hi) {
         for (int i = lo; i < hi; i++) {
             if (!valid[i] || region[i] || ratio[i] >= PENUMBRA_RATIO)