Replace zoom FFT peak search with a budget-bounded PyFFTW direct transform

Drop the coarse+fine zoom refinement, the SciPy FFT backend, and the
exact= audit path in favor of a single always-on full-transform peak
search (_peak_bins). Block size is now derived from a per-thread memory
budget (_fft_block_for/SRAS_FFT_PLAN_BUDGET_MB) instead of a fixed
constant, so the existing block-parallel PyFFTW pool stays memory-safe
at high pad factors without the zoom algorithm's bookkeeping. Also
removes the now-unused threadpoolctl dependency and the FFT backend
selector from the UI.

Also includes a pre-existing min_freq_mhz peak-search floor (excludes
bins below a caller-supplied frequency from the argmax) that was
already implemented and tested in the working tree.

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

tests/test_stored_cache.py

@@ -90,7 +90,7 @@ def no_fft(monkeypatch):
     """Make any real FFT work loud: returns a list that stays empty unless a
     peak search actually runs."""
     calls = []
-    for name in ("_peak_bins_direct", "_peak_bins_zoom"):
+    for name in ("_peak_bins",):
         original = getattr(compute, name)
 
         def spy(*args, _f=original, **kwargs):
@@ -147,7 +147,7 @@ def test_cache_is_stored_at_the_configured_pad(tmp_path, pad):
     gen.write(path, n_angles=2, seed=13, samples_per_frame=256)
     spf = SrasFile(str(path)).samples_per_frame
 
-    assert cache_file(str(path), "fft", True, "scipy", 0, pad) == ""
+    assert cache_file(str(path), "fft", True, 0, pad) == ""
     sras = SrasFile(str(path))
     assert sras.precomputed_pad_factor == pad, "pad factor survives the round trip"