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",

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,
                 )
             ]

beta_spectrum/output/csv_export.py

@@ -81,6 +81,18 @@ class CSVEmitter:
         if config.use_exchange:
             enabled.append("exchange")
 
+        # The header endpoint must describe the spectrum that is actually
+        # computed. Single mode with exactly one configured branch follows
+        # the branch (the top-level endpoint may be stale from the parent
+        # decay, e.g. after intensity-cutoff filtering).
+        basic_endpoint_MeV = config.endpoint_MeV
+        if (
+            not analyzer._is_multi
+            and config.branches is not None
+            and len(config.branches) == 1
+        ):
+            basic_endpoint_MeV = analyzer._unified_branches[0].endpoint_MeV
+
         parent_symbol = analyzer._element_symbol(config.Z_parent)
         daughter_symbol = analyzer._element_symbol(config.Z_daughter)
 
@@ -99,7 +111,7 @@ class CSVEmitter:
             f"# nuclide: {parent_symbol}{config.A_number} "
             f"-> {daughter_symbol}{config.A_number} "
             f"(Z={config.Z_parent}->{config.Z_daughter})",
-            f"# basic_endpoint: {config.endpoint_MeV * 1000:.1f} keV",
+            f"# basic_endpoint: {basic_endpoint_MeV * 1000:.1f} keV",
         ]
 
         if config.neutrino_mass_MeV > 0:

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