feat: v0.4.0 package restructuring — split into 4 modules

Separate monolithic beta_spectrum package into:
  beta_spectrum   — pure theory (components, spectrum, CLI, visualize)
  exp_data        — experimental data handling (spectrum, calibration,
                    fitters, corrections)
  fitter          — analysis framework (model, fit_engine, extractor,
                    result)
  beta_spectrum.visualize — plotting (BetaSpectrumAnalyzer)

Phase 1 — exp_data module:
  ExpSpectrum dataclass with metadata, normalization, serialization
  EnergyCalibrator (linear/quadratic calibration)
  GaussianFitter + PeakFitter for calibration sources
  DeadTimeCorrection, PileUpCorrection, BackgroundSubtractor
  43 new tests, ruff + mypy clean

Phase 2 — fitter module:
  SpectrumModel wraps BetaSpectrum + DetectorResponse with convolution
  SpectrumFitter with χ² minimization, covariance, profile likelihood,
  confidence intervals, robust loss functions
  CWExtractor + GVAExtractor for C(W) and g_V/g_A extraction
  AnalysisResult + FitSummary for physics-specific results
  CurveFitter general-purpose callable model fitter
  34 new tests, ruff + mypy clean

Phase 3 — clean up beta_spectrum:
  Remove cw_extractor.py and fitter.py (moved to fitter/)
  Clean __init__.py to export only theory classes
  Add backward-compat methods to CWExtractor
  Fix BetaSpectrumAnalyzer._get_branch_spectrum returning BetaSpectrum
  objects instead of numpy arrays (plot fix)

Phase 4 — package structure:
  Auto-discovered by setuptools (4 packages)
  All imports updated across 26 source files
  Black formatting applied
  322 tests pass (262 original + 60 new)
  ruff + mypy clean on all source files

Bug fixes:
  - Fix multi-branch spectrum plotting: universal component keys
    (Fermi, Screening, etc.) don't have per-branch prefix; fixed
    fallback lookup.
  - Fix negative phase space beyond endpoint: BetaSpectrum returns
    negative values for W > W0; clamp branch spectra to zero.

tests/fitter/test_extractor.py

@@ -0,0 +1,161 @@
+"""Tests for fitter.extractor."""
+
+import numpy as np
+
+from fitter.extractor import (
+    CWExtractor,
+    GVAExtractor,
+    CWExtractionResult,
+    GVAExtractionResult,
+)
+
+
+class TestCWExtractor:
+    """Test CWExtractor."""
+
+    def test_extract(self):
+        from beta_spectrum import SpectrumConfig
+
+        config = SpectrumConfig(
+            Z_parent=43,
+            Z_daughter=44,
+            A_number=99,
+            endpoint_MeV=0.294,
+        )
+        extractor = CWExtractor(config)
+
+        energies_keV = np.linspace(0.01, 0.29, 50)
+        measured = np.linspace(10.0, 0.1, 50)
+        errors = np.sqrt(measured)
+        model = np.linspace(9.5, 0.09, 50)
+
+        result = extractor.extract(
+            measured_counts=measured,
+            measured_errors=errors,
+            energies_keV=energies_keV,
+            model_counts=model,
+            endpoint_keV=294.0,
+        )
+
+        assert isinstance(result, CWExtractionResult)
+        assert len(result.cw_values) == 50
+        assert len(result.energies_keV) == 50
+        assert np.isclose(result.endpoint_keV, 294.0)
+        assert np.all(np.isfinite(result.cw_values))
+
+    def test_extract_with_zero_model(self):
+        from beta_spectrum import SpectrumConfig
+
+        config = SpectrumConfig(
+            Z_parent=43,
+            Z_daughter=44,
+            A_number=99,
+            endpoint_MeV=0.294,
+        )
+        extractor = CWExtractor(config)
+
+        energies_keV = np.linspace(0.01, 0.29, 50)
+        measured = np.ones(50) * 5.0
+        errors = np.ones(50) * 2.0
+        model = np.zeros(50)  # All zeros
+
+        result = extractor.extract(
+            measured_counts=measured,
+            measured_errors=errors,
+            energies_keV=energies_keV,
+            model_counts=model,
+            endpoint_keV=294.0,
+        )
+
+        # C(W) should be 1.0 where model is zero (default)
+        assert np.all(result.cw_values == 1.0)
+
+
+class TestGVAExtractor:
+    """Test GVAExtractor."""
+
+    def test_extract_from_curve_basic(self):
+        energies_W = np.linspace(1.1, 1.57, 100)
+        endpoint_W = 1.575  # ~294 keV
+        cw_values = np.ones_like(energies_W)  # C(W) = 1 for allowed
+
+        result = GVAExtractor.extract_from_curve(
+            cw_values=cw_values,
+            energies_W=energies_W,
+            endpoint_W=endpoint_W,
+        )
+
+        assert isinstance(result, GVAExtractionResult)
+        assert np.isclose(result.gv_eff, 1.0)
+        assert result.chi2_per_dof >= 0
+
+    def test_extract_from_curve_with_slope(self):
+        energies_W = np.linspace(1.1, 1.57, 100)
+        endpoint_W = 1.575
+        # C(W) with slight slope
+        cw_values = 1.0 + 0.1 * (energies_W - endpoint_W)
+
+        result = GVAExtractor.extract_from_curve(
+            cw_values=cw_values,
+            energies_W=energies_W,
+            endpoint_W=endpoint_W,
+        )
+
+        assert isinstance(result, GVAExtractionResult)
+        # g_A should be non-zero due to slope
+        assert abs(result.ga_eff) > 0
+
+    def test_extract_from_curve_insufficient_data(self):
+        energies_W = np.array([1.0, 1.5])
+        endpoint_W = 1.575
+        cw_values = np.array([1.0, 1.0])
+
+        result = GVAExtractor.extract_from_curve(
+            cw_values=cw_values,
+            energies_W=energies_W,
+            endpoint_W=endpoint_W,
+        )
+
+        assert isinstance(result, GVAExtractionResult)
+        assert result.chi2_per_dof == 0.0
+
+    def test_gva_summary(self):
+        result = GVAExtractionResult(
+            gv_eff=1.0,
+            gv_error=0.01,
+            ga_eff=0.5,
+            ga_error=0.02,
+            correlation=-0.3,
+            chi2_per_dof=1.2,
+        )
+        summary = result.summary()
+        assert "g_V" in summary
+        assert "g_A" in summary
+        assert "1.0" in summary
+        assert "0.5" in summary
+        assert "1.2" in summary
+
+
+class TestCWExtractionResult:
+    """Test CWExtractionResult methods."""
+
+    def test_kurie_plot(self):
+        energies_W = np.linspace(1.1, 1.57, 100)
+        endpoint_W = 1.575
+
+        result = CWExtractionResult(
+            energies_W=energies_W,
+            energies_keV=energies_W * 510.998950 - 510.998950,
+            cw_values=np.ones_like(energies_W),
+            cw_errors=np.zeros_like(energies_W),
+            flux=np.ones_like(energies_W) * 10.0,
+            flux_errors=np.ones_like(energies_W) * 3.0,
+            endpoint_W=endpoint_W,
+            endpoint_keV=294.0,
+        )
+
+        E_keV, kurie = result.kurie_plot()
+        assert len(E_keV) > 0
+        assert len(kurie) > 0
+        assert np.all(np.isfinite(kurie))
+        assert np.all(kurie >= 0)