Bragg integration: a CCD rotation sweep's sigmas at the measured gain, intensities in photons

The pre-scan measures the gain of an SMV/marCCD detector (background variance per unit mean) but the
integrator used it only for the background clip: sigma, var_bkg, the strong-spot flag (I/sigma >= 5)
and the profile-vs-box-sum guard (10 box sigmas) all treated one ADU as one photon, so on a Saturn
(g 4.7-5.8) the guard sat at ~4.5 true sigma. The new BraggIntegrationSettings::PixelGain (set by
rugnux from the same measurement, rotation only, at max(1, g)) takes every variance the engine forms
at the gain, and Finalize converts I, sigma, bkg and var_bkg to photons, which is what the merge's
variance model (var_bkg + I) assumes. CPU and CUDA engines; Metal runs the CPU code for this. Gain 1
(every photon-counting input, ADSC/marCCD whose measured ratio is below 1, and stills) is bit-identical.

Battery --only, 12 open-arm CCD/background-dominated sets vs the base build (987e812e2), GPU:
the six Saturn sets (g 4.7-5.8) all improve - guard fallbacks 4.1-17.3% -> 1.2-6.1%,
anomalous peak height 3mc4 x1.10, 3p85 x1.02, 3r6o x1.06, 5cc8 x1.03, 5uth x1.05, 5vml x1.04;
R_meas 0.880-0.993x on all six; R_model shell-scaled better on 5/6 (5cc8 +0.0005); ISa 5uth +1.5%,
5vml +15%, 3p85 +3%, the twinned 3mc4/3r6o -5%/-1%. 6cee, 6i3j, 6v2r, 7kcn, 8v4j, 9jzo unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CeEGgpKdfkCTJb3YNMJcu

tests/BraggIntegrationEngineGPUTest.cpp

@@ -150,13 +150,15 @@ double CompareCpuVsGpu(IntegratorMode mode, std::optional<float> bandwidth_fwhm,
                      float r1 = 0.0f, float r2 = 0.0f, float r3 = 0.0f,
                      OverlapMode overlap = OverlapMode::Off, float companion_dx = 0.0f,
                      bool clip_spots = false, float odd_partiality = 1.0f,
-                     const SpotFootprint *footprint = nullptr) {
+                     const SpotFootprint *footprint = nullptr, float pixel_gain = 1.0f) {
     DiffractionExperiment experiment =
             MakeExperiment(mode, bandwidth_fwhm, clip_nsigma, radial, DetJF(2), stencil_k, r1, r2, r3,
                            overlap);
-    if (footprint) {
+    {
         BraggIntegrationSettings settings = experiment.GetBraggIntegrationSettings();
-        settings.Footprint(*footprint);
+        if (footprint)
+            settings.Footprint(*footprint);
+        settings.PixelGain(pixel_gain);
         experiment.ImportBraggIntegrationSettings(settings);
     }
     const size_t width = experiment.GetXPixelsNum();
@@ -324,6 +326,16 @@ TEST_CASE("BraggIntegrationEngineGPU_MatchesCPU") {
         CompareCpuVsGpu(IntegratorMode::ProfileGaussian, std::nullopt, 4.0f, false, 8, 0.0f,
                         0.0f, 0.0f, 0.0f, OverlapMode::Off, 0.0f, false, 0.01f);
     }
+    // A CCD's gain: every variance at it, so the strong flag and the guard decide in photons, and the
+    // reflections leave in photons - computed independently in the two engines.
+    SECTION("ProfileGaussian pixel gain") {
+        CompareCpuVsGpu(IntegratorMode::ProfileGaussian, std::nullopt, 4.0f, false, 60, 0.0f,
+                        0.0f, 0.0f, 0.0f, OverlapMode::Off, 0.0f, false, 1.0f, nullptr, 4.0f);
+    }
+    SECTION("BoxSum pixel gain") {
+        CompareCpuVsGpu(IntegratorMode::BoxSum, std::nullopt, 4.0f, false, 60, 0.0f,
+                        0.0f, 0.0f, 0.0f, OverlapMode::Off, 0.0f, false, 1.0f, nullptr, 4.0f);
+    }
     SECTION("ProfileGaussian mono trim") { CompareCpuVsGpu(IntegratorMode::ProfileGaussian, std::nullopt, 0.0f); }
     // Unreadable pixels inside the signal disks themselves: the MINPK rescue keeps the reflection and
     // fits it over what is left, and the peak-loss rule throws back the ones that lost the profile's