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).

tests/quality/test_nuclear_data.py

@@ -5,6 +5,7 @@ import tempfile
 from pathlib import Path
 from typing import Any, Dict
 
+import numpy as np
 import pytest
 
 # ruff: noqa: E402, ANN201, ANN001
@@ -19,7 +20,7 @@ from beta_spectrum.nuclear_data import (  # noqa: E402
     validate_json_input,
     _parse_nuclide_symbol,
 )
-from beta_spectrum.spectrum import SpectrumConfig
+from beta_spectrum.spectrum import BetaSpectrum, SpectrumConfig
 
 
 def _has_paceENSDF() -> bool:  # noqa: ANN201
@@ -241,6 +242,52 @@ class TestDecayInfoToConfig:
         assert len(config.branches) == 1
         assert config.branches[0].intensity == 0.5
 
+    def test_cutoff_single_survivor_endpoint_follows_branch(self):
+        """A lone surviving branch must define the spectrum, not the stale
+        top-level endpoint inherited from the parent decay (D1)."""
+        info = DecayInfo(
+            parent_symbol="Cs137",
+            Z_parent=55,
+            Z_daughter=56,
+            A_number=137,
+            endpoint_keV=1176.0,
+            endpoint_MeV=1.176,
+            transition_type="F2",
+            forbiddenness_code="2F",
+            parent_J=7.0,
+            parent_pi=1,
+            half_life="y",
+            branches=[
+                BranchInfo(
+                    level_index=0,
+                    level_energy_keV=0.0,
+                    intensity=0.053,
+                    log_ft=5.0,
+                    transition_type="F2",
+                ),
+                BranchInfo(
+                    level_index=1,
+                    level_energy_keV=661.66,
+                    intensity=0.947,
+                    log_ft=5.0,
+                    transition_type="A",
+                ),
+            ],
+        )
+        cfg = decay_info_to_config(info, intensity_cutoff=0.9)
+        assert cfg.branches is not None
+        assert len(cfg.branches) == 1
+        lone_ep = cfg.branches[0].endpoint_MeV
+        assert lone_ep == pytest.approx(1.176 - 0.66166)
+        spectrum = BetaSpectrum.from_config(cfg)
+        assert not spectrum._is_multi
+        W, E = spectrum.get_energy_grid(cfg)
+        # Grid capped at the surviving branch endpoint, not 1.176 MeV.
+        assert float(E.max()) < lone_ep + 1e-12
+        values = spectrum(W)
+        assert np.all(np.isfinite(values))
+        assert np.all(values >= 0.0)
+
 
 # ---------------------------------------------------------------------------
 # get_decay_info_from_paceENSDF