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.

beta_spectrum/visualize/__init__.py

@@ -163,7 +163,14 @@ class BetaSpectrumAnalyzer:
     def _get_branch_spectrum(self, i: int) -> np.ndarray:
         """Get the spectrum array for branch i."""
         if i < len(self.spectrum.branch_spectra):
-            return self.spectrum.branch_spectra[i]
+            bs_obj = self.spectrum.branch_spectra[i]
+            # branch_spectra stores BetaSpectrum objects — call them to get arrays
+            if hasattr(bs_obj, "__call__"):
+                val = bs_obj(self.W)
+                # Clamp to zero beyond endpoint (phase space goes negative)
+                W0 = T_to_W(self._unified_branches[i].endpoint_MeV)
+                return np.where(self.W <= W0, val, 0.0)
+            return np.asarray(bs_obj)
         # Single-branch: the spectrum itself is the only branch
         return self.spectrum(self.W)
 
@@ -276,15 +283,19 @@ class BetaSpectrumAnalyzer:
 
         # Compute branch contributions (universal * intensity * branch_spectrum)
         branch_contribs = []
+        # Universal components are shared (no per-branch key prefix)
+        universal_names = [
+            "Fermi", "Screening", "FiniteSizeL0",
+            "ChargeDistributionU", "Exchange",
+        ]
         for i in range(self._n_branches):
             universal_product = np.ones_like(self.W)
-            for name in [
-                "Fermi", "Screening", "FiniteSizeL0",
-                "ChargeDistributionU", "Exchange",
-            ]:
+            for name in universal_names:
                 key = self._get_component_key(i, name)
                 if key in components:
                     universal_product *= components[key]
+                elif name in components:  # fallback: universal components
+                    universal_product *= components[name]
             bs = self._get_branch_spectrum(i)
             contrib = universal_product * self._unified_branches[i].intensity * bs
             branch_contribs.append(contrib)
@@ -348,15 +359,18 @@ class BetaSpectrumAnalyzer:
 
         # Panel 1: Total spectrum + per-branch spectra
         branch_contribs = []
+        universal_names = [
+            "Fermi", "Screening", "FiniteSizeL0",
+            "ChargeDistributionU", "Exchange",
+        ]
         for i in range(n_branches):
             universal_product = np.ones_like(self.W)
-            for name in [
-                "Fermi", "Screening", "FiniteSizeL0",
-                "ChargeDistributionU", "Exchange",
-            ]:
+            for name in universal_names:
                 key = self._get_component_key(i, name)
                 if key in components:
                     universal_product *= components[key]
+                elif name in components:
+                    universal_product *= components[name]
             bs = self._get_branch_spectrum(i)
             contrib = universal_product * self._unified_branches[i].intensity * bs
             branch_contribs.append(contrib)