Refinement: a Refit's solvent grid starts around where the context's last one ended

Logic change (deterministic), kept on evidence. Every Refit searched the whole k_sol/b_sol box - an 88-point
coarse pass of plain least squares, then the robust fine pass around its winner. ModelScaleBox (and
ModelScaleGPUPool::Fit / ModelScaleGPU::FitSolvent) take an optional start; with one, the coarse pass is
skipped and the fine pass is laid around it. Refinement passes the grid winner of the context's previous
Refit (before MoveOverallB moves the overall B into b_sol); it travels with a trial's fit, and a trial's first
Refit without one (Context::ForgetStarts, which also clears the likelihood's D start) searches the whole box,
so a trial still depends on nothing its slot ran before (side by side = one at a time, md5).

Evidence (GPU, -N 8): full refinement of the 14 parity sets from their placed models - 12 refined CIFs
identical (md5), 9ig7 and 6iu9 differ, final R-work/R-free equal to 4 digits but 6iu9 R-work 0.2911 -> 0.2910;
of the 5 timing cases 4 identical, 7d98 (cif start) R-free 0.3096 -> 0.3095; 5f6m MD starts 01-04 median R-free
0.1692 -> 0.1692; 6o2h on the CPU engine identical (md5). The score-only parity table is not touched (one Refit,
no start). refit.scale (TIMING sum): 5f6m 1.52 -> 1.10 s. Wall: 5reo 1.68 -> 1.53, 5f6m 2.43 -> 2.24, 9yl4 7.19 ->
7.02, 7d98 3.70 -> 3.65, 5f6m MD start 4.57 -> 4.22 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CeEGgpKdfkCTJb3YNMJcu
(cherry picked from commit ff285fb525ecf4b480698d4aafe212a34301a8fe)

image_analysis/structure_refinement/ModelScaleGPU.h

@@ -63,8 +63,9 @@ public:
     // (five or fewer reflections for the isotropic fit).
     // robust: the fine pass by robust least squares, as FitModelScale with ModelScaleBox::robust - the
     // reweighting on the device, the median of each fit's first residuals included.
+    // k_start, b_start: ModelScaleBox's - the fine pass around them, no coarse pass (negative: as FitModelScale).
     ModelSolventFit FitSolvent(const float2 *d_fcmol, const float2 *d_fmask, double b_hi = 80.0,
-                               bool robust = false);
+                               bool robust = false, double k_start = -1, double b_start = -1);
     // What a robust FitSolvent takes on the card beyond DeviceBytes(), reserved at its first call.
     static size_t RobustBytes(size_t max_points);