feat: experimental data calibration pipeline with ROOT I/O

- Fix chi2 calculation bug in calibration.py (remove incorrect
  coeffs[::-1] reversal in np.polyval)
- Fix broken beta_spectrum/cli.py (remove orphaned detector response
  add_argument calls and unused imports)
- Add ExpSpectrum.apply_calibration() and EnergyCalibrator.from_am241_spectrum()
- Add LivetimeDetermination class for pulser-based livetime
- Add SpectrumPipeline orchestration (read→subtract→calibrate→livetime→validate)
- Add ROOT I/O with PyROOT primary + uproot fallback
- Add bs_exp CLI with process, livetime, export subcommands
- Add exp_calibration notebook with pre-calibration visualization
- Add comprehensive tests (73 passing)
- Add bs_exp console script and [root] extra to pyproject.toml
- Remove obsolete detector_response module and unused notebooks

beta_spectrum/__init__.py

@@ -9,7 +9,6 @@ from .components.finite_size import FiniteSizeL0, ChargeDistributionU
 from .components.screening import ScreeningCorrection
 from .components.exchange import ExchangeCorrection
 from .components.radiative import RadiativeCorrection
-from .components.detector_response import DetectorResponse
 
 from .spectrum import BetaSpectrum, SpectrumConfig, BranchConfig
 from .visualize import BetaSpectrumAnalyzer
@@ -39,7 +38,6 @@ __all__ = [
     "ScreeningCorrection",
     "ExchangeCorrection",
     "RadiativeCorrection",
-    "DetectorResponse",
     "BetaSpectrum",
     "SpectrumConfig",
     "BetaSpectrumAnalyzer",