common: remove dead Reflection.completeness field

Reflection.completeness (spot-footprint fraction) was only ever set to its
1.0f default and never read anywhere - not in scaling/merge, CBOR, HDF5, or
the viewer. The box-sum integrator already rejects any partial-footprint
reflection outright (n_inner_valid == n_inner), so the intended "down-weight
clipped spots" role is already covered more strictly. Removed to reduce
confusion.

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

common/Reflection.h

@@ -26,7 +26,6 @@ struct Reflection {
     float dist_ewald;
     float rlp;
     float partiality;     // fraction of the reflection recorded in the sampled (rocking) slice
-    float completeness = 1.0f; // fraction of the spot footprint on live pixels (1 = not clipped by edge/gap/mask)
     float zeta;
     float image_scale_corr; // I_true = scaling_correction * I; scaling_correction = rlp / (partiality * image_scale)
     bool observed = false;