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/core.py

@@ -55,12 +55,20 @@ class BetaSpectrumAnalyzer:
         self._is_multi = spectrum._is_multi
         self._timestamp: Optional[str] = None
 
-        # Unified branch view: single-branch treated as one-branch multi-branch
+        # Unified branch view: single-branch treated as one-branch multi-branch.
+        # When config.branches carries exactly one branch, that branch (not the
+        # top-level decay fields) defines the endpoint and transition type.
         if not self._is_multi and self.spectrum.branch_spectra == []:
+            endpoint_MeV = config.endpoint_MeV
+            transition_type = config.transition_type
+            if config.branches is not None and len(config.branches) == 1:
+                lone = config.branches[0]
+                endpoint_MeV = float(lone.endpoint_MeV)
+                transition_type = str(lone.transition_type)
             self._unified_branches: List[_UnifiedBranch] = [
                 _UnifiedBranch(
-                    endpoint_MeV=config.endpoint_MeV,
-                    transition_type=config.transition_type,
+                    endpoint_MeV=endpoint_MeV,
+                    transition_type=transition_type,
                     intensity=1.0,
                 )
             ]