feat(detector): add detector response module, convolution, and integration

- Analytical detector response: gaussian, gaussian_tail, tikhonov models
- Low-energy tail modeling for charge collection effects
- Response matrix convolution via R^T · (Φ·ΔW)
- Declarative API: create_detector_from_config(), convolve_detector()
- BetaSpectrumAnalyzer.convolved_spectrum() for auto-convolution
- Integration tests: TestBetaSpectrumIntegration, TestDeclarativeDetectorResponse
- Documentation: docs/08-detector-response.md

beta_spectrum/__init__.py

@@ -9,6 +9,7 @@ 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, BetaSpectrumAnalyzer
 
@@ -27,9 +28,10 @@ __all__ = [
     "ScreeningCorrection",
     "ExchangeCorrection",
     "RadiativeCorrection",
+    "DetectorResponse",
     "BetaSpectrum",
     "SpectrumConfig",
     "BetaSpectrumAnalyzer",
 ]
 
-__version__ = "0.1.2"
+__version__ = "0.2.0"