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/__init__.py

@@ -13,8 +13,6 @@ from .components.detector_response import DetectorResponse
 
 from .spectrum import BetaSpectrum, SpectrumConfig, BranchConfig
 from .visualize import BetaSpectrumAnalyzer
-from .fitter import CurveFitter, FitConfig, FitResult
-from .cw_extractor import CWExtractor, CWExtractionResult, GVAExtractionResult
 from .nuclear_data import (
     DecayInfo,
     BranchInfo,
@@ -46,12 +44,6 @@ __all__ = [
     "SpectrumConfig",
     "BetaSpectrumAnalyzer",
     "BranchConfig",
-    "CurveFitter",
-    "FitConfig",
-    "FitResult",
-    "CWExtractor",
-    "CWExtractionResult",
-    "GVAExtractionResult",
     "DecayInfo",
     "BranchInfo",
     "get_decay_info_from_paceENSDF",