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

tests/quality/test_nuclear_data.py

@@ -161,9 +161,12 @@ class TestJsonInput:
         path = self._write_json(data)
         try:
             loaded = load_json_input(str(path))
-            # Defaults should be filled in
-            assert loaded["detector_model"] == "gaussian"
-            assert loaded["detector_sigma_a_keV"] == 1.0
+            # Defaults should be filled in. The detector_* schema keys were
+            # removed together with the detector_response module; JSON
+            # defaults carry spectrum physics only.
+            assert "detector_model" not in loaded
+            assert loaded["neutrino_mass_MeV"] == 0.0
+            assert loaded["transition_type"] == "F2"
         finally:
             path.unlink()