refactor: remove stale detector schema keys and single-mode duplicates

- DEFAULT_JSON_SCHEMA: drop the 8 detector_* keys left over from the
  removed detector_response module; CLI dry-run drops the dead
  'Detector: not configured (Phase 4)' line
- CSV export: single-mode no longer writes branch_1_PhaseSpace /
  branch_1_Radiative duplicates next to the bare PhaseSpace/Radiative
  columns; per-branch core factors only in multi-branch mode
- Visualizer: debug panels fall back to the unprefixed core-factor
  columns for single-mode CSVs instead of skipping them
- Spectrum ylabel now states the actual units: normalized spectrum
  [MeV^-1], not 'Normalized Counts / keV'
- tests: defaults assert detector keys gone; column layout pinned for
  single vs multi; panel and ylabel assertions

beta_spectrum/output/csv_export.py

@@ -211,14 +211,20 @@ class CSVEmitter:
 
         if csv_verbosity == "full":
             # Per-branch component columns (factorized ingredients).
-            for i in range(n_branches):
-                for comp_name in ["PhaseSpace", "Radiative"]:
-                    key = analyzer._get_component_key(i, comp_name)
-                    if key in components:
-                        data[f"branch_{i + 1}_{comp_name}"] = components[key]
+            # Only multi-branch mode carries per-branch factors: in single
+            # mode the bare PhaseSpace/Radiative columns below already
+            # describe the single spectrum, and prefixed duplicates would
+            # silently disagree with them once renamed.
+            if analyzer._is_multi:
+                for i in range(n_branches):
+                    for comp_name in ["PhaseSpace", "Radiative"]:
+                        key = analyzer._get_component_key(i, comp_name)
+                        if key in components:
+                            data[f"branch_{i + 1}_{comp_name}"] = components[key]
 
             # Universal components (no prefix): Fermi plus universal
-            # higher-order corrections.
+            # higher-order corrections. In single mode the core factors
+            # (PhaseSpace, Radiative) also appear here unprefixed.
             for name, values in components.items():
                 if not name.startswith("branch_"):
                     data[name] = values