feat: add paceENSDF integration and CLI interface (Track B1)

- Add nuclear_data.py module for paceENSDF integration and JSON input
- Add DecayInfo and BranchInfo dataclasses for parsed nuclear data
- Implement get_decay_info_from_paceENSDF() to retrieve decay parameters
- Implement load_json_input() and json_to_config() for custom JSON input
- Add create_config_from_source() unified interface for both sources
- Add bs_pnpi CLI with --nuclide, --input, --output, --plot, and detector options
- Add 23 comprehensive tests for nuclear_data module
- Add sample JSON input file
- Update __init__.py and pyproject.toml with new exports and entry point
- Add paceENSDF as optional dependency

beta_spectrum/__init__.py

@@ -14,6 +14,16 @@ from .components.detector_response import DetectorResponse
 from .spectrum import BetaSpectrum, SpectrumConfig, BetaSpectrumAnalyzer
 from .fitter import CurveFitter, FitConfig, FitResult
 from .cw_extractor import CWExtractor, CWExtractionResult, GVAExtractionResult
+from .nuclear_data import (
+    DecayInfo,
+    BranchInfo,
+    get_decay_info_from_paceENSDF,
+    decay_info_to_config,
+    create_config_from_source,
+    load_json_input,
+    json_to_config,
+    DEFAULT_JSON_SCHEMA,
+)
 
 __all__ = [
     "ALPHA",
@@ -40,6 +50,14 @@ __all__ = [
     "CWExtractor",
     "CWExtractionResult",
     "GVAExtractionResult",
+    "DecayInfo",
+    "BranchInfo",
+    "get_decay_info_from_paceENSDF",
+    "decay_info_to_config",
+    "create_config_from_source",
+    "load_json_input",
+    "json_to_config",
+    "DEFAULT_JSON_SCHEMA",
 ]
 
 __version__ = "0.3.0"