fft: add irfft as the inverse real FFT (fixes #176)

[axiom-of-choice]

Fixes #176. Adds aneforge.fft.irfft(X_re, X_im, N) as the inverse real FFT, the natural completion of the existing rfft.

Approach: thin wrapper over the existing ifft_plan machinery, as the issue suggests. Runs the on-engine inverse FFT over the Hermitian-symmetric spectrum and returns the real part. ifft already applies the 1/N normalization, and for a Hermitian spectrum the imaginary part is ~0 (numpy.fft.irfft also returns only the real part), so the real-signal contract holds with no new kernel or plan.

Changes

• aneforge/fft.py: irfft function + export in __all__.
• aneforge/fft.py _selftest: round-trip irfft(rfft(x)) ~= x across N=128 and N=512, plus a numpy.fft.irfft oracle over the half spectrum.
• tests/test_fft.py (new, requires_ane like test_fft2.py): 8 tests covering fft/ifft/rfft/irfft against numpy, the round-trip, the numpy oracle, and that irfft returns a real array.

Checks run

• On-device (M2 Pro / macOS): _selftest PASS, tests/test_fft.py 8 passed.
• Off-device: pytest -m "not requires_ane" 407 passed, 2 skipped. ruff check, the pylint 2-space gate, pyright aneforge/fft.py tests/test_fft.py (0 errors), and compileall all clean.

CI runs the off-device subset; the transform itself dispatches, so the on-device _selftest and tests/test_fft.py were run locally and pass on M2 Pro.

[sbryngelson]

Verified on device (M5): all 8 tests in the new file pass. Thanks for adding tests/test_fft.py --
the FFT module had no pytest coverage at all before this.

Two small things before merge.

The docstring should say irfft takes the FULL length-N spectrum, the way this module's rfft
returns it, and not the N//2+1 half spectrum that numpy.fft.irfft expects. The name invites the
numpy reading, and the test itself has to slice Xr[:N // 2 + 1] to build its reference, so the
mismatch is already visible in the PR.

The new test file also has no trailing newline.

Both, as a patch:

diff --git a/aneforge/fft.py b/aneforge/fft.py
index 4350d40..d9cfb1c 100644
--- a/aneforge/fft.py
+++ b/aneforge/fft.py
@@ -292,7 +292,10 @@ def rfft(x_real, N: int):
 def irfft(X_re, X_im, N: int):
   """Inverse real FFT of a Hermitian-symmetric spectrum on the ANE; returns the real time-domain
   signal of length N (the imag part is ~0 by Hermitian symmetry, and numpy.fft.irfft also
-  returns only the real part)."""
+  returns only the real part).
+
+  Takes the FULL length-N spectrum, as `rfft` returns it -- not the N//2+1 half spectrum that
+  numpy.fft.irfft expects."""
   x_re, _ = ifft_plan(N)(X_re, X_im)
   return x_re
 
diff --git a/tests/test_fft.py b/tests/test_fft.py
index 666d9c3..2be98c5 100644
--- a/tests/test_fft.py
+++ b/tests/test_fft.py
@@ -60,4 +60,4 @@ def test_irfft_returns_real_only():
   x = rng.standard_normal(256).astype(np.float32)
   Xr, Xi = agfft.rfft(x, 256)
   back = agfft.irfft(Xr, Xi, 256)
-  assert np.iscomplexobj(np.asarray(back)) is False
\ No newline at end of file
+  assert np.iscomplexobj(np.asarray(back)) is False