FFBIDXIndexerCPU: the fast-feedback indexer on the CPU, used where there is no GPU

A port of the fast-feedback indexer's CUDA kernels to plain C++/Eigen: the same half-sphere and
rotation samples, the same scores and the same 32 output cells for Refine(). The loops are
reorganised so the inner one runs over samples and vectorises (GCC/Clang, no intrinsics), and the
cell search separates the rotation about the candidate vector out of the per-spot terms. The
vector refinement reproduces the library's warp sums - partial sums its shuffle reduction leaves in
the other lanes carry into the next round - and its float arithmetic (FMA contraction, the
Fibonacci points in float), because the candidates searched depend on it: on the library's 27 test
spot lists and 4 synthetic stills the CPU and GPU best cells agree to ~1e-6 relative, the top-32
sets in 30 of 31 cases completely.

FFBIDX now resolves to the GPU library where a GPU is visible and to the CPU port otherwise (no
GPU, --no-gpu, or a build without CUDA); Auto takes FFBIDX for stills whenever a cell is known,
also without a GPU, instead of FFTW. With a GPU nothing changes (a stills set and a rotation set
reproduce the baseline exactly). Without a GPU the stills indexer pool gets one indexer per worker
instead of eight, since the CPU indexer holds no card memory.

Single core, synthetic still, loaded 5950X: ~8 ms per call at 30 spots, ~15 ms at 80, ~35 ms at
250; 16 threads ~1300/700/330 calls/s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CeEGgpKdfkCTJb3YNMJcu
(cherry picked from commit 9a8eb8669d1be6c3ec16db6786280affd6830ee5)

image_analysis/indexing/IndexerThreadPool.cpp

@@ -11,6 +11,7 @@
 
 #include "../common/CUDAWrapper.h"
 #include "../common/Logger.h"
+#include "FFBIDXIndexerCPU.h"
 
 #ifdef JFJOCH_USE_CUDA
 #include "FFBIDXIndexer.h"
@@ -173,6 +174,8 @@ static std::unique_ptr<Indexer> MakeIndexer(IndexingAlgorithmEnum algorithm, con
             return std::make_unique<FFBIDXIndexer>();
     }
 #endif
+    if (algorithm == IndexingAlgorithmEnum::FFBIDX)
+        return std::make_unique<FFBIDXIndexerCPU>();
 #ifdef JFJOCH_USE_FFTW
     if (algorithm == IndexingAlgorithmEnum::FFTW)
         return std::make_unique<FFTIndexerCPU>(settings);
@@ -446,7 +449,7 @@ IndexerResult IndexerThreadPool::Run(const DiffractionExperiment &experiment, co
     const bool servable =
         (algorithm == IndexingAlgorithmEnum::FFT    && have_gpu &&
              (requested == IndexingAlgorithmEnum::Auto || requested == IndexingAlgorithmEnum::FFT)) ||
-        (algorithm == IndexingAlgorithmEnum::FFBIDX && have_gpu &&
+        (algorithm == IndexingAlgorithmEnum::FFBIDX &&
              (requested == IndexingAlgorithmEnum::Auto || requested == IndexingAlgorithmEnum::FFBIDX)) ||
         (algorithm == IndexingAlgorithmEnum::FFTW   && fftw_built &&
              ((requested == IndexingAlgorithmEnum::Auto && !have_gpu) || requested == IndexingAlgorithmEnum::FFTW));
@@ -457,8 +460,6 @@ IndexerResult IndexerThreadPool::Run(const DiffractionExperiment &experiment, co
                   "always uses the GPU FFT indexer here; select FFT or Auto, or run FFTW on a CPU-only node.";
         else if (algorithm == IndexingAlgorithmEnum::FFT && !have_gpu)
             msg = "FFT is the GPU indexer but no GPU is available. Select FFTW or Auto for CPU indexing.";
-        else if (algorithm == IndexingAlgorithmEnum::FFBIDX && !have_gpu)
-            msg = "FFBIDX is a GPU indexer but no GPU is available. Select FFTW or Auto for CPU indexing.";
         else if (algorithm == IndexingAlgorithmEnum::FFTW)
             msg = "FFTW (CPU) indexing was requested but this build has no FFTW indexer.";
         else