new background remove algoritm

tests/test_processing.py

@@ -14,6 +14,11 @@ from rfg_adc_plotter.processing.calibration import (
     recalculate_calibration_c,
     save_calib_envelope,
 )
+from rfg_adc_plotter.processing.background import (
+    load_fft_background,
+    save_fft_background,
+    subtract_fft_background,
+)
 from rfg_adc_plotter.processing.fft import (
     build_positive_only_centered_ifft_spectrum,
     build_symmetric_ifft_spectrum,
@@ -101,6 +106,28 @@ class ProcessingTests(unittest.TestCase):
             with self.assertRaises(ValueError):
                 load_calib_envelope(path)
 
+    def test_fft_background_roundtrip_and_rejects_non_1d_payload(self):
+        background = np.asarray([0.5, 1.5, 2.5], dtype=np.float32)
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            path = os.path.join(tmp_dir, "fft_background")
+            saved_path = save_fft_background(path, background)
+            loaded = load_fft_background(saved_path)
+            self.assertTrue(saved_path.endswith(".npy"))
+            self.assertTrue(np.allclose(loaded, background))
+
+            invalid_path = os.path.join(tmp_dir, "fft_background_invalid.npy")
+            np.save(invalid_path, np.zeros((2, 2), dtype=np.float32))
+            with self.assertRaises(ValueError):
+                load_fft_background(invalid_path)
+
+    def test_subtract_fft_background_clamps_negative_residuals_to_zero(self):
+        signal = np.asarray([1.0, 2.0, 3.0], dtype=np.float32)
+        background = np.asarray([1.0, 1.5, 5.0], dtype=np.float32)
+        subtracted = subtract_fft_background(signal, background)
+
+        self.assertTrue(np.allclose(subtracted, np.asarray([0.0, 0.5, 0.0], dtype=np.float32)))
+        self.assertTrue(np.allclose(subtract_fft_background(signal, signal), 0.0))
+
     def test_apply_working_range_crops_sweep_to_selected_band(self):
         freqs = np.linspace(3.3, 14.3, 12, dtype=np.float64)
         sweep = np.arange(12, dtype=np.float32)