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/exp_data/test_spectrum.py

@@ -0,0 +1,141 @@
+"""Tests for exp_data.spectrum."""
+
+import numpy as np
+import pytest
+
+from exp_data.spectrum import ExpSpectrum
+
+
+class TestExpSpectrumBasic:
+    """Basic ExpSpectrum construction and properties."""
+
+    def test_construct_with_errors(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0, 2.0]),
+            counts=np.array([10.0, 20.0, 30.0]),
+            errors=np.array([3.16, 4.47, 5.48]),
+        )
+        assert spectrum.n_channels == 3
+        np.testing.assert_array_equal(spectrum.energies, [0.0, 1.0, 2.0])
+
+    def test_automatic_poisson_errors(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0, 2.0]),
+            counts=np.array([10.0, 20.0, 30.0]),
+        )
+        expected = np.sqrt([10.0, 20.0, 30.0])
+        np.testing.assert_allclose(spectrum.errors, expected)
+
+    def test_zero_counts_get_error_one(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0]),
+            counts=np.array([0.0, 100.0]),
+        )
+        assert spectrum.errors[0] == 1.0  # sqrt(1) for zero counts
+
+    def test_shape_mismatch_raises(self):
+        with pytest.raises(AssertionError):
+            ExpSpectrum(
+                energies=np.array([0.0, 1.0]),
+                counts=np.array([10.0, 20.0, 30.0]),
+            )
+
+    def test_negative_energies_raises(self):
+        with pytest.raises(AssertionError):
+            ExpSpectrum(
+                energies=np.array([-1.0, 0.0]),
+                counts=np.array([10.0, 20.0]),
+            )
+
+    def test_negative_counts_raises(self):
+        with pytest.raises(AssertionError):
+            ExpSpectrum(
+                energies=np.array([0.0, 1.0]),
+                counts=np.array([10.0, -5.0]),
+            )
+
+    def test_total_counts(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0, 2.0]),
+            counts=np.array([10.0, 20.0, 30.0]),
+        )
+        assert spectrum.total_counts == 60.0
+
+    def test_energy_range(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.5, 1.5, 2.5]),
+            counts=np.array([10.0, 20.0, 30.0]),
+        )
+        assert spectrum.energy_range == (0.5, 2.5)
+
+    def test_normalize(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0, 2.0]),
+            counts=np.array([10.0, 20.0, 30.0]),
+        )
+        norm = spectrum.normalize()
+        integral = np.trapezoid(norm, spectrum.energies)
+        assert np.isclose(integral, 1.0, rtol=1e-10)
+
+
+class TestExpSpectrumDeadTime:
+    """Dead-time correction on ExpSpectrum."""
+
+    def test_no_correction_when_dead_time_zero(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0]),
+            counts=np.array([10.0, 20.0]),
+        )
+        corrected = spectrum.apply_dead_time_correction()
+        np.testing.assert_array_equal(corrected.counts, spectrum.counts)
+
+    def test_correction_applied(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0]),
+            counts=np.array([10.0, 20.0]),
+            dead_time=0.1,
+        )
+        corrected = spectrum.apply_dead_time_correction()
+        expected = spectrum.counts / 0.9
+        np.testing.assert_allclose(corrected.counts, expected, rtol=1e-10)
+        assert corrected.dead_time == 0.0
+
+    def test_live_time_corrected_rate(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0]),
+            counts=np.array([10.0, 20.0]),
+            dead_time=0.1,
+            live_time=10.0,
+        )
+        rate = spectrum.live_time_corrected_rate
+        assert rate is not None
+        # Total counts / (live_time * (1 - dead_time))
+        expected = 30.0 / (10.0 * 0.9)
+        assert np.isclose(rate, expected)
+
+
+class TestExpSpectrumSerialization:
+    """Dict round-trip for ExpSpectrum."""
+
+    def test_to_from_dict(self):
+        spectrum = ExpSpectrum(
+            energies=np.array([0.0, 1.0, 2.0]),
+            counts=np.array([10.0, 20.0, 30.0]),
+            dead_time=0.05,
+            live_time=100.0,
+            source="Tc99",
+            run_id="run_042",
+            date="2025-01-15",
+            metadata={"detector": "MMC", "temperature": "25 mK"},
+        )
+
+        recovered = ExpSpectrum.from_dict(spectrum.to_dict())
+
+        np.testing.assert_allclose(recovered.energies, spectrum.energies)
+        np.testing.assert_allclose(recovered.counts, spectrum.counts)
+        assert recovered.dead_time == spectrum.dead_time
+        assert recovered.live_time == spectrum.live_time
+        assert recovered.source == spectrum.source
+        assert recovered.run_id == spectrum.run_id
+        assert recovered.date == spectrum.date
+        assert recovered.metadata == spectrum.metadata