fix(spectrum): lone branch defines endpoint in single-branch mode

When config.branches carries exactly one branch (e.g. after
--intensity-cutoff filtering), the branch endpoint/transition define
the spectrum, energy grid, analyzer unified branch view and CSV
header. The stale top-level endpoint inherited from the parent decay
no longer leaks into physics (proven wrong on Cs137: spectrum
extended to 1176 keV while the lone survivor ends at 514 keV).

beta_spectrum/output/__init__.py

@@ -12,9 +12,6 @@ from .core import BetaSpectrumAnalyzer, _UnifiedBranch
 from .csv_export import CSVEmitter
 from .visualization import Visualizer
 
-# Re-export the main class for backward compatibility
-BetaSpectrumAnalyzer = BetaSpectrumAnalyzer
-
 __all__ = [
     "BetaSpectrumAnalyzer",
     "CSVEmitter",