Feat: Use FFTW linear convolution for CPU fast-mode PSF resolve

Replace the nested spatial global convolution in fast_psf_accumulator_resolve with a reusable double-precision FFTW linear convolution on the CPU PSF backend.

- Add the private src/fast_psf_fftw.{c,h} module: zero-padded R2C/C2R plans, cached kernel spectrum and planar scratch, exact 1/(Pwidth*Pheight) and 1/N^2 normalization, (R,R) crop, and additive HDR output.
- Keep the previous nested loops as fast_psf_accumulator_resolve_spatial_reference for tests/benchmarks only; it is not a runtime fallback.
- Cache the circular row spans on FastPsfAccumulator and report one-time plan, kernel transform, scratch, and per-frame stage timings.
- Require fftw3_omp for CPU builds; HIP and dummy builds do not link FFTW.
- Namespace test/helper binaries by spacetime and build tag, and reject make test / psf-capture for non-CPU backends.
- Add tests/test_fast_psf_fftw.c (FFTW versus spatial), tests/benchmark_fast_psf_fftw.c, an FFTW CLI smoke check, and the 2026-09-25 benchmark record.

build.md

@@ -10,9 +10,13 @@ The default CPU build needs:
 - A C11 compiler and OpenMP runtime (for example, GCC with libgomp).
 - GNU Make.
 - libpng headers and library for PNG output.
+- `pkg-config`, used to locate and link FFTW.
+- FFTW 3 built with OpenMP, discovered through `pkg-config fftw3_omp`
+  (`sci-libs/fftw[openmp]` on Gentoo). This is required for the CPU fast-mode
+  fast convolution; HIP and dummy PSF builds do not link FFTW.
 
-Optional dependencies are CFITSIO and `pkg-config` for HDR/FITS output, and
-HIP/ROCm with `hipcc` and a compatible GPU for HIP PSF accumulation.
+Optional dependencies are CFITSIO for HDR/FITS output, and HIP/ROCm with
+`hipcc` and a compatible GPU for HIP PSF accumulation.
 
 ## CPU build
 
@@ -181,7 +185,7 @@ the test executable; run it separately:
 
 ```sh
 make PSF_BACKEND=hip SPACETIME=minkowski hip-psf-test
-./build/Release/test_hip_psf
+./build/Release/obj/minkowski/standard_sink1_hip/test_hip_psf
 ```
 
 For rendering with real stellar data, continue with [README.md](README.md#prepare-the-stellar-catalog)