merge: deltaCChalf uses the deterministic HalfForImage split, not an RNG

MergeOnTheFly::DeltaCChalfReject assigned its CC1/2 half-sets from a seeded
std::mt19937 drawn in image (call) order, whereas the actual merge, the
rotation merge, the GPU path and the R-free flags all split with the
deterministic HalfForImage(image_id) splitmix64 hash. So the deltaCChalf was
measured on a different half-partition than the reported CC1/2, and was
order-dependent (a reordered outcomes vector gave different halves).

Use HalfForImage(i) - i is the image's stable index, the same image_id AddImage
merges with - so deltaCChalf now reflects the exact CC1/2 split the statistics
report, order-independently.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

image_analysis/scale_merge/Merge.cpp

@@ -474,9 +474,6 @@ std::vector<char> MergeOnTheFly::DeltaCChalfReject(const std::vector<Integration
     std::unordered_map<uint64_t, Acc> acc;
     std::vector<int> img_half(outcomes.size(), 0);
 
-    std::mt19937 lrng{2026061600u};
-    std::bernoulli_distribution hd{0.5};
-
     // ---- pass 1: accumulate half-set sums, record each image's half ----
     auto contribution = [&](const Reflection &r, uint64_t &key, double &wI, double &w) -> bool {
         if (generator.IsSystematicallyAbsent(r)) return false;
@@ -493,7 +490,9 @@ std::vector<char> MergeOnTheFly::DeltaCChalfReject(const std::vector<Integration
     };
 
     for (size_t i = 0; i < outcomes.size(); ++i) {
-        const int half = hd(lrng) ? 1 : 0;
+        // Same deterministic half-set as the merge (HalfForImage), so deltaCChalf is measured on the
+        // exact CC1/2 split the statistics report - not an independent, order-dependent RNG draw.
+        const int half = HalfForImage(static_cast<int64_t>(i));
         img_half[i] = half;
         for (const auto &r : outcomes[i].reflections) {
             uint64_t key; double wI, w;