HostWorkerPool: half the budget, MR placements reserve 16 B per grid point, output unchanged

Without a GPU (macOS), rc176's worker pool - 1/3 of the 12.9 GB budget on a 16 GB Mac - let only two
molecular-replacement placements run at once, as each reserved 32 B per grid point (2.1 GB on 4nwv)
against a measured ~0.95 GB. MR rescoring went from 38.6 s to 69 s and 4nwv's wall time from 402 s
(budget off) to 444 s.

On the M6 Mac mini (16 GB), 4nwv, two runs each in alternating order on a quiet machine:
- old build 234774ff0: 418.4 s, 20.0-21.3 GiB peak footprint, 576-682k swapouts
- rc176 as shipped: 444.2 s, 14.5-15.2 GiB, 39-66k swapouts
- this change: 402.4 s, 15.5-15.8 GiB, 66-93k swapouts
- budget switched off: 401.7 s, 21.8-23.3 GiB, 268-433k swapouts
9zlo/9s02/9gjx: 71.9/79.0/82.8 s (old build 73.6/82.2/83.6), no swap. p.hkl byte-identical in every
run: the pool decides only what runs at once.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018My7tqt8a4LyKKjP3FhScT

image_analysis/structure_refinement/MolecularReplacement.cpp

@@ -2777,7 +2777,7 @@ MolecularReplacementResult search_model(const SearchModel &model, const std::vec
                 // gigabyte or more of grids on a large cell, so each reserves them against the host's worker
                 // pool first (HostWorkerPool), as a null replicate does.
                 const size_t placement_bytes = get_gpu_count() == 0
-                        ? MODEL_FACTORS_BYTES_PER_GRID_POINT * ModelGridPoints(cell, &sg, d_min)
+                        ? MODEL_PLACEMENT_BYTES_PER_GRID_POINT * ModelGridPoints(cell, &sg, d_min)
                         : 0;
                 for (First *f : to_refine)
                     running.push_back(std::async(std::launch::async, [&, f] {