Add soft per-spot quality weighting for adaptive spot detection

Add --soft-weight (implies --adaptive-spots): give every detected spot a
continuous quality weight in (0,1] and keep the highest-weight spots rather than
the brightest, so a deliberately loose detector self-cleans -- bright ice / salt
/ jet blobs and single-pixel noise no longer evict faint clean Bragg spots from
the max-spots cut.

The weight is a product of dimensionless gates (AdaptiveSpotFinderCPU::ApplyWeights,
computed against the per-ring background the adaptive finder already builds): a
logistic ramp in the spot's SNR and a soft size band (rises from one pixel,
plateaus, falls for oversized ice/salt/streak blobs). It carries on
DiffractionSpot -> SpotToSave and is consumed by FilterSpotsByCount, which ranks
by {non-ice, weight, intensity} when requested and by intensity otherwise, so the
classic and FPGA paths are unchanged.

Honest result: on the serial-stills battery this is index-rate-NEUTRAL. The
weighted ranking only changes the outcome when the spot count exceeds the
max-spots cap and the weight disagrees with intensity in a way that affects
indexing; the adaptive detectors already produce clean spot lists and the weak
sets sit under the cap, so re-ranking is a wash there (and a wash, not a
regression, on the one set that floods). Its intended benefit -- robustness to
ice/jet-contaminated frames and to a loosened detector -- is not exercised by
this battery; kept opt-in as the substrate for that.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

common/DiffractionSpot.cpp

@@ -85,6 +85,7 @@ std::optional<SpotToSave> DiffractionSpot::Export(const DiffractionGeometry &geo
         .lattice = -1,
         .image = image_num,
         .d_A = d,
+        .weight = weight,
         .ice_ring = false,
         .indexed = false
     };