Choose min-pix-per-spot adaptively per image for serial-stills indexing

For stills indexing the minimum-pixels-per-spot filter is now chosen per image
instead of being fixed: the frame is indexed at min-pix 3/2/1 and the setting that
maximises indexed-spot count weighted by indexed fraction (n_indexed^2 / n_total)
is kept, then integrated once at that min-pix. The fraction factor keeps a smaller
min-pix's extra spots only when the lattice actually explains them, so strong frames
retain their real weak spots (extending resolution) while noise-flooded frames stay
strict.

The mode is selected by the presence of --min-pix-per-spot, now optional
(SpotFindingSettings::min_pix_per_spot is std::optional<int64_t>): omit it for the
adaptive per-image path, give a value to force a fixed min-pix. It applies only to
the stills indexing path -- rotation indexing builds one global lattice and keeps a
fixed min-pix, and the online receiver and the FPGA host path always carry a concrete
value, so neither changes. IndexAndRefine::ProcessImage now returns whether the frame
indexed, to drive the per-image selection.

Exposed in the jfjoch_viewer spot-finding settings (adaptive-threshold and
adaptive-min-pix checkboxes, each greying out the control it overrides); the broker
uses neither.

Validated on the full rotation regression battery (no regression) and the whole
serial-stills target battery at full image count.

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

image_analysis/IndexAndRefine.cpp

@@ -510,13 +510,14 @@ IndexAndRefine::DetermineRefineAnalyze(DataMessage &msg, const SpotFindingSettin
     return outcome;
 }
 
-void IndexAndRefine::ProcessImage(DataMessage &msg,
+bool IndexAndRefine::ProcessImage(DataMessage &msg,
                                   const SpotFindingSettings &spot_finding_settings,
                                   BraggPrediction &prediction,
                                   const BraggIntegrateFn &integrate) {
     auto outcome = DetermineRefineAnalyze(msg, spot_finding_settings);
     if (outcome && spot_finding_settings.quick_integration)
         QuickPredictAndIntegrate(msg, spot_finding_settings, prediction, integrate, *outcome);
+    return outcome.has_value();
 }
 
 bool IndexAndRefine::IndexFrameOnly(DataMessage &msg, const SpotFindingSettings &spot_finding_settings) {