Replace FFTW with fortnum fft_c2c and drop the FFTW dependency (4/5)

[krystophny]

Merge order

Stack sequence: #29 -> #30 -> #31 -> #32 -> #33.

Each PR is now based on main individually, so its diff is cumulative against main and shares code with its stack neighbors. Merge in the sequence above. Once a predecessor merges into main, the next PR rebases onto it and its diff shrinks to its own increment.

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Fourth in the fortnum migration stack (base: the C-quadrature PR). Removes MEPHIT's FFTW usage.

Dependency removed: FFTW. The fft_t one-dimensional forward transform in src/mephit_util.f90 moves onto fortnum's fft_c2c, which matches the FFTW sign convention (sign = -1, unnormalized). The type now owns an allocatable samples buffer instead of FFTW-allocated pointers and an opaque plan; apply transforms a copy in place so the caller's samples survive, then extracts and normalizes the requested mode range exactly as before. src/fftw3.f90, the cmake/FindFFTW.cmake module, and the empty FFTW_LIBRARIES link entry are removed. find_package(FFTW) was never invoked in this repo, so FFTW_LIBRARIES expanded to nothing and the FFTW symbols from src/fftw3.f90 were unresolved at executable link time; dropping that source fixes the link.

Verification

Full build at this tip, including the executables that did not link on the earlier tips and on main:

$ cmake -S . -B build -G Ninja && cmake --build build
[27/30] Linking CXX shared library lib/libmephit.so
[28/30] Linking C executable bin/mephit_run.x
[29/30] Linking Fortran executable bin/mephit_post.x
[30/30] Linking Fortran executable bin/mephit_test.x

MEPHIT references no FFTW symbol and uses fortnum's transform instead:

$ nm -DuC build/lib/libmephit.so | grep -ic fftw
0
$ nm -DC build/lib/libmephit.so | grep -i fft_c2c
0000000000120020 T __fortnum_fft_MOD_fft_c2c
$ ldd build/bin/mephit_run.x | grep -i fftw || echo "no fftw in mephit_run.x deps"
no fftw in mephit_run.x deps

Before this PR the executable link failed:

$ cmake --build build
/usr/bin/ld: lib/libmephit.so: undefined reference to `fftw_plan_dft_1d'
/usr/bin/ld: lib/libmephit.so: undefined reference to `fftw_execute_dft'
...
collect2: error: ld returned 1 exit status