Fix method ambiguity with FFTW.jl for plan_rfft/plan_brfft

[pankgeorg]

With FFTW.jl and FFTA loaded in the same session, plan_rfft(::Vector{Float64}, ::Int) — and therefore rfft(x) — is a method ambiguity:

MethodError: plan_rfft(::Vector{Float64}, ::Int64) is ambiguous.
  plan_rfft(X::StridedArray{Float64, N}, region; ...) @ FFTW
  plan_rfft(x::AbstractArray{T, N}, region::Union{Int64, NTuple{N, Int64} where N, AbstractVector{Int64}}; ...) @ FFTA

FFTW's method is more specific in the array argument, FFTA's in the region argument, so neither wins. Same for plan_brfft (d::Integer vs len::Int). plan_fft/plan_bfft were fine because their region argument is unannotated.

This leaves region unannotated on the AbstractFFTs.plan_rfft/plan_brfft entry points (as plan_fft already does) and normalises it in an internal _plan_rfft/_plan_brfft, so that FFTW's StridedArray methods are strictly more specific and take over when loaded — which is what AbstractFFTs intends (ref #66 for the discussion of that behaviour). FFTA's methods remain reachable for non-strided arrays.

A regression test loads FFTW.jl and checks rfft/irfft/fft work and that no plan_* ambiguities exist. It runs in a subprocess, because using FFTW in the test process would make every other test in the suite exercise FFTW instead of FFTA. This adds FFTW as a test-only dependency; happy to drop the test if that's unwanted.

Found while writing the benchmark suite in #128, where FFTA and FFTW are loaded together.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.64%. Comparing base (7aeb327) to head (2f58ee0).

Files with missing lines	Patch %	Lines
src/plan.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   98.80%   98.64%   -0.17%     
==========================================
  Files           5        5              
  Lines         585      589       +4     
==========================================
+ Hits          578      581       +3     
- Misses          7        8       +1     

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