integration: gate the background-ring sigma-clip to stills (bandwidth>0)

The 76e88b0f sigma-clip (reject ring pixels above mean+3*sqrt(mean))
de-biases bandwidth-streaked high-resolution stills, but it ran on all
data. On rotation (no streaks) it clips legitimate high background pixels
and biases the mean low, slightly inflating weak intensities and hurting
the anomalous signal. Gate it to bandwidth>0 (stills), matching how the
814dff34 radial-profile change is already gated.

Rotation lysozyme (self-scaled, smooth-g, -A): anomalous S-peak 0.84x ->
0.86x of XDS (SD_MET 11.71 -> 11.94, CL_CL 1.28x -> 1.25x), ISa unchanged.
Stills (bandwidth set) are byte-identical (clip still applies).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

image_analysis/bragg_integration/ProfileIntegrate2D.cpp

@@ -29,7 +29,8 @@ struct Rough {
 
 template<class T>
 Rough BoxSum(const T *image, size_t xpixel, size_t ypixel, int64_t special, int64_t saturation,
-             const Reflection &r, float r1_sq, float r2_sq, float r3_sq, float r3, float min_sigma_ratio) {
+             const Reflection &r, float r1_sq, float r2_sq, float r3_sq, float r3, float min_sigma_ratio,
+             bool apply_bkg_clip) {
     Rough out;
     const int64_t x0 = std::max<int64_t>(0, std::floor(r.predicted_x - r3 - 1.0));
     const int64_t x1 = std::min<int64_t>(xpixel - 1, std::ceil(r.predicted_x + r3 + 1.0));
@@ -64,7 +65,9 @@ Rough BoxSum(const T *image, size_t xpixel, size_t ypixel, int64_t special, int6
     // (or a neighbour) leaks into the annulus and biases the mean high, which over-subtracts and drives
     // the merged high-resolution intensities negative; rejecting pixels above mean + 3*sqrt(mean) removes
     // that contamination (and barely touches a clean Poisson background, where ~0.1% exceed the cut).
-    {
+    // Stills-only: on rotation (no bandwidth streak) it clips legitimate high background pixels and biases
+    // the mean low, slightly hurting weak (anomalous) intensities, so it is gated off there.
+    if (apply_bkg_clip) {
         const double thr = out.bkg + 3.0 * std::sqrt(std::max(out.bkg, 1.0));
         double s = 0.0;
         int n = 0;
@@ -95,6 +98,9 @@ std::vector<Reflection> ProfileIntegrateInternal(const DiffractionExperiment &ex
     const float r2 = settings.GetR2(), r2_sq = r2 * r2;
     const float r3 = settings.GetR3(), r3_sq = r3 * r3;
     const float min_sigma_ratio = settings.GetMinimumSigmaInRegardsToI();
+    // The background-ring sigma-clip de-biases bandwidth-streaked stills but can clip legitimate
+    // background on rotation (no streaks) and bias it low; gate it to stills (a bandwidth is set).
+    const bool apply_bkg_clip = experiment.GetBandwidthFWHM().value_or(0.0f) > 0.0f;
     const int R = static_cast<int>(std::ceil(r2));   // profile grid half-size
     const int G = 2 * R + 1, GG = G * G;
     auto idx = [G, R](int dx, int dy) { return (dy + R) * G + (dx + R); };
@@ -108,7 +114,7 @@ std::vector<Reflection> ProfileIntegrateInternal(const DiffractionExperiment &ex
     double inv_d2_min = std::numeric_limits<double>::max(), inv_d2_max = 0.0;
     for (size_t i = 0; i < npredicted; ++i) {
         rough[i] = BoxSum<T>(ptr, xpixel, ypixel, special, saturation, predicted[i],
-                             r1_sq, r2_sq, r3_sq, r3, min_sigma_ratio);
+                             r1_sq, r2_sq, r3_sq, r3, min_sigma_ratio, apply_bkg_clip);
         if (rough[i].ok && predicted[i].d > 0.0f) {
             const double inv_d2 = 1.0 / (predicted[i].d * predicted[i].d);
             inv_d2_min = std::min(inv_d2_min, inv_d2);