rugnux: model validation does the same work in less time

--model validation (battery-only for users) was 18% of the battery's time. Every
number it produces is unchanged to the bit (p.mtz, maps, placed model and every
model-validation line of the report md5/diff-identical on 11 open sets); only
when and where the work runs changes:

- The bulk-solvent grid fit (FitModelScale, most of the CPU time) fits each
  solvent pair on a copy of gemmi::Scaling's target that takes
  |Fcalc + k_sol exp(-b_sol s^2) Fmask| once per pair instead of at every
  solver evaluation; same expressions, same types (new test checks a grid
  point against gemmi's own Scaling fit with ==).
- Fcalc density and the solvent mask are made on two threads; the model's
  structure factors beside the GPU engine reservation.
- The indexing probe fits the relabellings concurrently.
- The null's replicates run beside the real model's placement (they start
  from a snapshot of the model as read); one GPU engine per replicate plus
  one for the real fit instead of a cap of 4 (engines are interchangeable
  and deterministic).
- The 2mFo-DFc, mFo-DFc and anomalous maps are made and written
  concurrently; the placed model is written beside the reflection files.
- A rigid-body zone whose solvent-mask grid needs gemmi's shrink is sent to
  the CPU when the engines are reserved (ModelMaskGPU::ShrinkIsNoOp), instead
  of failing on the GPU and validating everything again on the CPU - the
  same CPU result, without the wasted first attempt.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SVmAWnzCmRKAXVUCdc4iNi

tests/ModelMaskGPUTest.cpp

@@ -214,6 +214,9 @@ TEST_CASE("ModelMaskGPU_RefusesAGridTheShrinkWouldChange", "[ModelValidation][gp
     ModelMaskGPU mask(stream, fine.point_count());
     CHECK_NOTHROW(mask.SetGrid(MaskGridDescription(coarse), ops));
     CHECK_THROWS(mask.SetGrid(MaskGridDescription(fine), ops));
+    // The same answer ahead of time, which is what sends such a grid to the CPU before any engine is made.
+    CHECK(ModelMaskGPU::ShrinkIsNoOp(MaskGridDescription(coarse)));
+    CHECK_FALSE(ModelMaskGPU::ShrinkIsNoOp(MaskGridDescription(fine)));
 }
 
 #endif