fix(nuclear_data): use measured transition endpoints per branch

paceENSDF stores the measured beta endpoint energy of each transition
(ENSDF column 7). The parser instead derived branch endpoints as
base-endpoint minus level energy with the base taken from an arbitrary
first-listed branch, so Co60 produced negative branch endpoints and a
hard ValueError (bs_pnpi Co60 exited 1 regardless of cutoff).

- BranchInfo carries the measured endpoint_MeV
- overall endpoint = max measured endpoint over all branches
- decay_info_to_config prefers measured endpoints; the subtraction is
  retained only as a legacy fallback for hand-built DecayInfo objects
- Co60/Tc99/Cs137 endpoint tests + CLI dry-run/export tests (D3)

tests/integration/test_cli_flows.py

@@ -163,6 +163,23 @@ class TestCLIPaceENSDF:
         assert exc.value.code == 1
         assert not (tmp_path / "tc99deg.csv").exists()
 
+    def test_paceensdf_co60_dry_run_multi_branch(self, capsys: Any) -> None:
+        """Co60 parses into a healthy multi-branch config (D3): no crash,
+        all branch endpoints positive."""
+        main(["Co60", "--dry-run"])
+        out = capsys.readouterr().out
+        assert "Multi-branch mode: 3 branches" in out
+
+    def test_paceensdf_co60_export(self, tmp_path: Any) -> None:
+        out = str(tmp_path / "co60.csv")
+        main(["Co60", "--output", out])
+        df = read_export(out)
+        area = np.trapezoid(df["total_spectrum"], df["energy_MeV"])
+        assert np.isclose(area, 1.0, rtol=1e-3)
+        eps = header_lines_of(out)
+        assert any("E₀=1492.0" in h for h in eps)
+        assert not any("excluded" in h for h in eps)
+
     def test_cli_builds_paceensdf_config_via_library(
         self, monkeypatch: Any, capsys: Any
     ) -> None:

tests/quality/test_nuclear_data.py

@@ -294,6 +294,44 @@ class TestDecayInfoToConfig:
 # ---------------------------------------------------------------------------
 
 
+@pytest.mark.slow
+@pytest.mark.requires_ensdf
+@pytest.mark.skipif(not _has_paceENSDF(), reason="paceENSDF not installed")
+class TestPaceENSDFBranchEndpoints:
+    """Per-branch endpoints are the measured transition endpoint energies
+    (ENSDF column 7), not base-endpoint minus level energy subtractions
+    with a base taken from an arbitrary first branch (D3)."""
+
+    def _branch_endpoints(self, nuclide: str) -> list[float]:
+        info = get_decay_info_from_paceENSDF(nuclide, "beta_minus")
+        cfg = decay_info_to_config(info)
+        assert cfg.branches is not None
+        return sorted(
+            (b.endpoint_MeV for b in cfg.branches),
+            reverse=True,
+        )
+
+    def test_co60_branch_endpoints_positive(self) -> None:
+        # Measured endpoints: 1492.0, 670.0, 317.88 keV (levels above the
+        # first branch's endpoint previously produced negative values and
+        # a hard ValueError from BranchConfig validation).
+        eps = self._branch_endpoints("Co60")
+        assert eps == pytest.approx([1.492, 0.670, 0.31788], abs=1e-6)
+        assert all(e > 0 for e in eps)
+
+    def test_tc99_branch_endpoints(self) -> None:
+        eps = self._branch_endpoints("Tc99")
+        assert eps == pytest.approx([0.2975, 0.2080], abs=1e-6)
+
+    def test_cs137_branch_endpoints(self) -> None:
+        eps = self._branch_endpoints("Cs137")
+        assert eps == pytest.approx([1.176, 0.89213, 0.51403], abs=1e-6)
+
+    def test_overall_endpoint_is_max_branch_endpoint(self) -> None:
+        info = get_decay_info_from_paceENSDF("Co60", "beta_minus")
+        assert info.endpoint_MeV == pytest.approx(1.492, abs=1e-6)
+
+
 @pytest.mark.requires_ensdf
 @pytest.mark.skipif(not _has_paceENSDF(), reason="paceENSDF not installed")
 class TestPaceENSDFIntegration: