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

exp_data/__init__.py

@@ -6,20 +6,37 @@ experimental spectra from detector readouts.
 """
 
 from .spectrum import ExpSpectrum
-from .calibration import EnergyCalibrator
+from .calibration import EnergyCalibrator, CalibrationResult
 from .fitters import GaussianFitter, PeakFitter
 from .corrections import (
     DeadTimeCorrection,
     PileUpCorrection,
     BackgroundSubtractor,
 )
+from .livetime import LivetimeDetermination, LivetimeResult
+from .pipeline import (
+    SpectrumPipeline,
+    PipelineResult,
+    process_spectrum,
+    process_run_sequence,
+)
+from .root_io import write_spectrum, read_spectrum
 
 __all__ = [
     "ExpSpectrum",
     "EnergyCalibrator",
+    "CalibrationResult",
     "GaussianFitter",
     "PeakFitter",
     "DeadTimeCorrection",
     "PileUpCorrection",
     "BackgroundSubtractor",
+    "LivetimeDetermination",
+    "LivetimeResult",
+    "SpectrumPipeline",
+    "PipelineResult",
+    "process_spectrum",
+    "process_run_sequence",
+    "write_spectrum",
+    "read_spectrum",
 ]