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/spectrum.py

@@ -95,6 +95,21 @@ class SpectrumConfig:
     )
 
 
+def _effective_single_fields(config: SpectrumConfig) -> Tuple[float, str]:
+    """Resolve (endpoint_MeV, transition_type) for single-branch mode.
+
+    When config.branches carries exactly one branch (e.g. after intensity
+    cutoff filtering removed all but one), that branch defines the spectrum:
+    its endpoint and transition type win over the (possibly stale) top-level
+    fields inherited from the parent decay. With branches=None the top-level
+    fields are used as before.
+    """
+    if config.branches is not None and len(config.branches) == 1:
+        lone = config.branches[0]
+        return float(lone.endpoint_MeV), str(lone.transition_type)
+    return float(config.endpoint_MeV), str(config.transition_type)
+
+
 class BetaSpectrum:
     """
     Precise beta spectrum calculator.
@@ -259,7 +274,8 @@ class BetaSpectrum:
         conditional.
         """
         components: List[SpectrumComponent] = []
-        W0 = float(T_to_W(config.endpoint_MeV))
+        eff_endpoint, eff_transition = _effective_single_fields(config)
+        W0 = float(T_to_W(eff_endpoint))
 
         # Core spectrum components (0-order): always included.
         # Convert neutrino mass from MeV to natural units
@@ -267,7 +283,7 @@ class BetaSpectrum:
         components.append(
             PhaseSpace(
                 W0=W0,
-                transition_type=config.transition_type,
+                transition_type=eff_transition,
                 m_nu=m_nu_natural,
                 logger=logger,
             )
@@ -419,6 +435,11 @@ class BetaSpectrum:
         """
         if self._is_multi and self.branches:
             max_endpoint = max(b.endpoint_MeV for b in self.branches)
+        elif config.branches is not None and len(config.branches) == 1:
+            # Single-mode with exactly one configured branch: the branch
+            # endpoint defines the grid, not the (possibly stale) top-level
+            # endpoint from the parent decay.
+            max_endpoint = float(config.branches[0].endpoint_MeV)
         else:
             max_endpoint = config.endpoint_MeV