rugnux: .hkl holds the unmerged scaled fulls on rotation data

The SHELX HKLF 4 file now has one record per full reflection - its partials summed,
the per-frame scale and every correction applied, sigma(I) as the merge weighted it -
at the index it was measured at, not averaged with its equivalents: the chemical
crystallographer's convention, so SHELXL computes Rint and Rsigma itself. Outliers
the merge rejected and fulls beyond its resolution cut are left out; no batch column
(it would select a BASF scale in SHELXL). The engine hands the fulls back only for a
merge that may be written (RotationScaleMerge::SetExportScaledFulls), so the search
merges, the pre-pass and the P1 cross-check carry no copy; the fulls follow the same
relabelling as the merged reflections (merge_to_written). Stills keep the merged
file. --mode scale writes the unmerged form too.

Validation: p.mtz md5 unchanged on myob/cytc/thau (GPU). SHELXL on the same runs,
merged-old vs unmerged-new (COD models, harness /data/tmp/sm_shared):
aspirin 20 keV Rint 0 -> 0.071, Rsigma 0.036 -> 0.043, R1 0.0964 -> 0.0969,
wR2 0.312 -> 0.310, GooF 1.53 -> 1.47; HEPES 20 keV Rint 0 -> 0.163, Rsigma 0.057
-> 0.068, R1 0.0903 -> 0.0897, wR2 0.318 -> 0.263, GooF 1.57 -> 1.10; the "input
data appear to be merged" warning is gone.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K5K8jvPPbmCrbqnWkddTuB

tests/WriteReflectionsTest.cpp

@@ -4,6 +4,7 @@
 #include <catch2/catch_all.hpp>
 
 #include <filesystem>
+#include <fstream>
 #include <string>
 #include <cstring>
 
@@ -185,3 +186,31 @@ TEST_CASE("Unmerged MTZ: built on several workers, the same rows as on one", "[w
         CHECK(parallel.batches.back().number == serial.batches.back().number);
     }
 }
+
+TEST_CASE("SHELX .hkl of unmerged fulls: fixed 3I4,2F8.2, scaled to fit, 0 0 0 terminator",
+          "[write_reflections][portable]") {
+    // Two fulls of the same reflection and a Friedel mate: written as measured, nothing averaged. The
+    // largest |I| or sigma is put at 9999.00; an unusable full (sigma not positive) is left out.
+    const std::vector<ScaledFull> fulls{
+            {1, 2, 3, 200000.0f, 2000.0f},
+            {-1, -2, -3, 100000.0f, 1200.0f},
+            {1, 2, 3, -50.0f, 20.0f},
+            {4, 5, 6, 10.0f, 0.0f},
+    };
+    const auto path = (std::filesystem::temp_directory_path() / "jfjoch_scaled_fulls_test.hkl").string();
+    WriteShelxHklReflections(fulls, path, 2);
+
+    std::ifstream in(path);
+    std::vector<std::string> lines;
+    for (std::string line; std::getline(in, line);)
+        lines.push_back(line);
+    std::filesystem::remove(path);
+
+    REQUIRE(lines.size() == 4);
+    for (const auto &line : lines)
+        CHECK(line.size() == 28);
+    CHECK(lines[0] == "   1   2   3 9999.00   99.99");
+    CHECK(lines[1] == "  -1  -2  -3 4999.50   59.99");
+    CHECK(lines[2] == "   1   2   3   -2.50    1.00");
+    CHECK(lines[3] == "   0   0   0    0.00    0.00");
+}