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/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: