--model: the bulk-solvent b_sol ceiling follows the model's atomic B

The model scale searched b_sol only over 10-80 A^2. That is the range
Fokine & Urzhumtsev report for models at ordinary B, but on low-resolution
entries whose deposited B is 200-400 A^2 the fit sat on the 80 A^2 ceiling
(and k_sol on its 0.1 floor): a solvent edge far sharper than the atoms it
lies against, reaching too deep in resolution. R-free came out 0.05-0.10
above REFMAC (0 cycles) and phenix.model_vs_data on the same MTZ, model
and flags.

The ceiling is now max(80, 2 x the model's mean atomic B) (SolventBMax),
for the reported fit and for the rigid-body placement's per-zone solvent
(CPU, GPU and the host fallback alike). The grid keeps eight b_sol values
across a wider box, so it costs no more points; the default box, and so
every model with mean B <= 40 A^2, is bit-identical to before.

Measured (10 open-arm sets with --model, GPU build, vs the same base):
  high-B set, 7.0 A:  R-free 0.462 -> 0.364, b_sol 80 -> 379 (REFMAC 0.366,
                      REFMAC's own b_sol ~371; model_vs_data 0.291);
                      the rigid-body step now commits
  high-B set, 3.9 A:  0.410 -> 0.331, b_sol 80 -> 388, k_sol 0.10 -> 0.225
                      (REFMAC 0.328, model_vs_data 0.314); null 10.1 -> 14.2 sigma
  B~200 set, 3.2 A:   0.408 -> 0.402, b_sol 80 -> 120; enantiomorph label
                      decision unchanged (27.2 -> 25.8 sigma)
  7 other sets (mean B 16-57): bit-identical R, k_sol, b_sol, placement.
p.hkl md5 and the MTZ reflection data are identical to the base on all 10.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

image_analysis/structure_refinement/ModelValidation.cpp

@@ -760,7 +760,7 @@ ModelValidationResult Validate(const std::vector<MergedReflection> &merged,
         gemmi::Scaling<float> scaling(ucell, sg);
         scaling.use_solvent = true;
         scaling.prepare_points(out.fmodel, fobs_work, &ms.fmask);
-        FitModelScale(scaling, {}, nthreads);
+        FitModelScale(scaling, {.b_hi = SolventBMax(ms.st.models[0])}, nthreads);
         scaling.scale_data(out.fmodel, &ms.fmask);   // out.fmodel now holds the scaled, solvent-corrected Fmodel
         out.k_sol = scaling.k_sol;
         out.b_sol = scaling.b_sol;