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)

beta_spectrum/nuclear_data.py

@@ -96,6 +96,10 @@ class BranchInfo:
         log(ft) value for this branch.
     transition_type : str
         Forbiddenness classification for this branch (A, F1, F1U, F2, ...).
+    endpoint_MeV : float
+        Measured beta endpoint energy of this transition in MeV (ENSDF
+        column 7). 0.0 means "not provided": consumers fall back to
+        base-endpoint minus level-energy subtraction.
     """
 
     level_index: int
@@ -103,6 +107,7 @@ class BranchInfo:
     intensity: float
     log_ft: Optional[float]
     transition_type: str = "A"
+    endpoint_MeV: float = 0.0
 
 
 def _parse_nuclide_symbol(symbol: str) -> Tuple[str, int, int]:
@@ -384,24 +389,19 @@ def get_decay_info_from_paceENSDF(
 
     # Extract Q-value from ground state branch (state[7] = endpoint energy in keV)
     # and half-life from halflife_data
-    endpoint_keV = 0.0
     half_life_str = "unknown"
 
+    # The spectrum endpoint is the maximum measured transition endpoint
+    # across all branches (ENSDF column 7, keV). Selecting the ground
+    # state branch or the first branch is wrong whenever those branches
+    # are absent or non-maximal (e.g. Co60, whose first listed branch
+    # endpoint is below higher-level transition endpoints).
+    endpoint_keV = 0.0
     if beta_data:
-        for k, v in beta_data.items():
+        for _k, v in beta_data.items():
             for state in v:
-                if state[0] == 0:  # ground state branch
-                    endpoint_keV = float(state[7])
-                    break
-            if endpoint_keV > 0:
-                break
-
-        # No ground state branch — use first available branch
-        if endpoint_keV == 0.0:
-            for k, v in beta_data.items():
-                if v and len(v) > 0:
-                    endpoint_keV = float(v[0][7])
-                    break
+                if len(state) > 7 and state[7] is not None:
+                    endpoint_keV = max(endpoint_keV, float(state[7]))
 
     if halflife_data:
         for k, v in halflife_data.items():
@@ -447,6 +447,11 @@ def get_decay_info_from_paceENSDF(
                 intensity=float(state[9]) / 100.0,
                 log_ft=float(state[11]) if state[11] is not None else None,
                 transition_type=branch_transition_type,
+                endpoint_MeV=(
+                    float(state[7]) / 1000.0
+                    if len(state) > 7 and state[7] is not None
+                    else 0.0
+                ),
             )
             branches.append(branch)
 
@@ -497,12 +502,19 @@ def decay_info_to_config(
     SpectrumConfig
         Configuration ready for BetaSpectrum.from_config().
     """
-    # Convert BranchInfo -> BranchConfig for SpectrumConfig
+    # Convert BranchInfo -> BranchConfig for SpectrumConfig. Per-branch
+    # endpoints are the measured transition endpoint energies when
+    # available; the base-endpoint minus level-energy subtraction is only
+    # a legacy fallback for hand-built DecayInfo objects without them.
     branches: Optional[List[BranchConfig]] = None
     if info.branches:
         branches = [
             BranchConfig(
-                endpoint_MeV=info.endpoint_MeV - b.level_energy_keV / 1000.0,
+                endpoint_MeV=(
+                    b.endpoint_MeV
+                    if b.endpoint_MeV > 0.0
+                    else info.endpoint_MeV - b.level_energy_keV / 1000.0
+                ),
                 transition_type=b.transition_type,
                 intensity=b.intensity,
             )
@@ -615,8 +627,13 @@ def load_json_input(filepath: str) -> Dict[str, Any]:
     if not path.exists():
         raise FileNotFoundError(f"Input file not found: {filepath}")
 
-    with open(path, "r") as f:
-        data = json.load(f)
+    try:
+        with open(path, "r", encoding="utf-8") as f:
+            data = json.load(f)
+    except OSError as exc:
+        raise FileNotFoundError(f"Input file not readable: {filepath}") from exc
+    except json.JSONDecodeError as exc:
+        raise ValueError(f"Invalid JSON in input file {filepath}: {exc}") from exc
 
     # Fill in defaults for missing optional fields
     for key, default in DEFAULT_JSON_SCHEMA.items():