tests: hold the integer coordinate conversion to the module map

ConvertToImageCoordinates now transforms the photon-weighted sums instead of
the centroid, which is only equivalent because a module's raw -> image map is
a signed axis swap plus an integer translation. Check that against the map
itself on every module of a detector whose modules do not share an
orientation, and either side of the 256-column multipixel gaps where the
translation changes. A wrong sign or a dropped gap term on any single module
would otherwise only show up as mispositioned spots on that module.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/DiffractionSpotTest.cpp

@@ -3,6 +3,7 @@
 
 #include <catch2/catch_all.hpp>
 #include "../common/DiffractionSpot.h"
+#include "../common/RawToConvertedGeometry.h"
 
 TEST_CASE("DiffractionSpot_ConvertToImageCoordinates", "[LinearAlgebra][Coord]") {
     DiffractionExperiment experiment(DetJF(4, 2, 8, 36, true));
@@ -20,3 +21,30 @@ TEST_CASE("DiffractionSpot_ConvertToImageCoordinates", "[LinearAlgebra][Coord]")
     REQUIRE(spot_1.RawCoord().y == Catch::Approx(CONVERTED_MODULE_LINES * 2 + 36 - 1 - 300.25 - 2));
     REQUIRE(spot_1.Count() == 12 * 2 + 4 * 2);
 }
+
+// ConvertToImageCoordinates transforms the photon-weighted SUMS in integers rather than converting
+// the centroid, which is only equivalent because the module map is a signed axis swap plus an
+// integer translation. Hold it to the map it is standing in for, on every module of a detector
+// whose modules do not all share an orientation, and either side of the 256-column multipixel gaps
+// where the translation changes.
+TEST_CASE("DiffractionSpot_ConvertMatchesModuleMap", "[LinearAlgebra][Coord]") {
+    DiffractionExperiment experiment(DetJF(8, 2, 8, 36, true));
+
+    for (uint16_t module = 0; module < experiment.GetModulesNum(); module++) {
+        for (const auto [col, line] : {std::pair<uint32_t, uint32_t>{3, 5},
+                                       {255, 255}, {256, 256}, {700, 300}, {1020, 505}}) {
+            INFO("module " << module << " pixel " << col << "," << line);
+            // Two pixels of unequal weight, so the centroid is a genuine fraction rather than a
+            // pixel centre - a whole-pixel centroid would not notice a lost fractional part.
+            DiffractionSpot spot(col, line, 7);
+            spot += DiffractionSpot(col, line + 1, 3);
+            const Coord raw = spot.RawCoord();
+
+            spot.ConvertToImageCoordinates(experiment, module);
+            const Coord expected = RawToConvertedCoordinate(experiment, module, raw);
+            REQUIRE(spot.RawCoord().x == Catch::Approx(expected.x));
+            REQUIRE(spot.RawCoord().y == Catch::Approx(expected.y));
+            REQUIRE(spot.Count() == 10);
+        }
+    }
+}