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

pyproject.toml

@@ -15,11 +15,14 @@ dependencies = [
     "numpy",
     "scipy",
     "matplotlib",
-    "pandas"
+    "pandas",
+    "nbformat>=5.10.4",
+    "nbconvert>=7.17.1",
 ]
 
 [project.optional-dependencies]
 ensdf = ["paceENSDF"]
+root = ["root"]  # PyROOT (requires system ROOT installation)
 dev = [
     "pytest",
     "nbmake",
@@ -34,6 +37,7 @@ dev = [
 
 [project.scripts]
 bs_pnpi = "beta_spectrum.cli:main"
+bs_exp = "exp_data.exp_cli:main"
 
 [tool.setuptools.packages.find]
 where = ["."]