conform FFT to project lint gates; rename test to algorithms_fft

- clang-format applied; clang-tidy clean under the project config
  (real fixes: path-derived header guard, concatenated namespace,
  unsigned shift, sign-safe comparisons, array::at for runtime
  indices, missing includes, const; justified NOLINT blocks for
  kernel-by-design categories: raw scratch pointer arithmetic,
  FFT-structured loop bounds, recursive sub-plans, math notation)
- fix -Werror=missing-braces in to_sizes_ (double-braced std::array)
- rename test/fft.cpp -> test/algorithms_fft.cpp: the fftw adaptor
  suite already owns the fft.cpp.x target name (CMP0002)

pre-push lanes green so far: g++ Debug 99/99, clang++ libc++
ASan/UBSan 101/101, nvcc CUDA 128/128 (test compiled as CUDA),
nvc++ 101/101, clang-format gate.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: alfC

include/boost/multi/algorithms/fft.NOTES.md

@@ -6,7 +6,7 @@ non-obvious **library idioms** discovered while writing it, and a concrete
 **future-work** list.
 
 - Implementation: [`fft.hpp`](./fft.hpp)
-- Tests: [`../../../../test/fft.cpp`](../../../../test/fft.cpp) (auto-registered by the `test/*.cpp` glob)
+- Tests: [`../../../../test/algorithms_fft.cpp`](../../../../test/algorithms_fft.cpp) (auto-registered by the `test/*.cpp` glob)
 
 ---
 
@@ -173,7 +173,7 @@ else is gathered interleaved with the copy loop order chosen from the strides.
 
 Verified two independent ways:
 
-- Against a direct O(N²) reference DFT in `test/fft.cpp`: radix and mixed sizes
+- Against a direct O(N²) reference DFT in `test/algorithms_fft.cpp`: radix and mixed sizes
   2…2048 (including all pow-2 stage combinations), Bluestein sizes 67, 101,
   134 = 2·67, 331, 1009; plus round-trips (1-D/2-D/3-D/4-D and the 65536
   six-step path), DC = sum, 2-D = composition of 1-D, 4-D = composition of