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_gui.py

@@ -223,6 +223,46 @@ def test_threshold_change_recomputes(ctx):
         f"masking zeroed some pixels ({n_zero} of {win._current_image.size})"
 
 
+def test_highlight_masked_pixels(ctx):
+    """Masked (below-threshold) pixels are drawn as NaN - filled with a
+    highlight color, separate from the normal colormap - so they can't be
+    mistaken for a real, possibly-low-frequency pixel; unchecking restores
+    the old behavior where both blend into the same plain 0."""
+    win = ctx.win
+    assert win.chk_highlight_masked.isChecked(), "on by default"
+    dc4 = win._dc_cache[(0, CH4_IDX)]
+    expect_masked = dc4 < win.spin_threshold_mv.value()
+    assert expect_masked.any() and not expect_masked.all(), \
+        "fixture threshold should mask some but not all pixels"
+
+    calls = []
+    orig = win.image_canvas.show_image
+
+    def spy(img, *a, **kw):
+        calls.append((np.array(img, copy=True), kw.get("bad_color")))
+        return orig(img, *a, **kw)
+
+    with patch.object(win.image_canvas, "show_image", side_effect=spy):
+        win._redraw_image(win._current_image)
+    shown, bad_color = calls[-1]
+    assert bad_color is not None, "highlight color set while checkbox is on"
+    assert np.array_equal(np.isnan(shown), expect_masked), \
+        "NaN exactly where DC4 is below threshold, nowhere else"
+
+    win.chk_highlight_masked.setChecked(False)
+    calls.clear()
+    with patch.object(win.image_canvas, "show_image", side_effect=spy):
+        win._redraw_image(win._current_image)
+    shown2, bad_color2 = calls[-1]
+    assert bad_color2 is None, "no highlight color once unchecked"
+    assert not np.isnan(shown2).any(), "unchecked: no pixel pulled out to NaN"
+    assert np.array_equal(shown2, win._current_image), \
+        "unchecked: displayed array is the raw, unmodified image"
+
+    win.chk_highlight_masked.setChecked(True)
+    pump(60)
+
+
 def test_bg_sub_toggle(ctx):
     """bg-sub no longer gates the display: it only affects a future live
     compute for an angle with nothing cached yet, or an explicit batch