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/nuclear_data.py

@@ -554,14 +554,6 @@ DEFAULT_JSON_SCHEMA = {
     "transition_type": "F2",
     "e_step_MeV": 0.001,
     "neutrino_mass_MeV": 0.0,
-    "detector_model": "gaussian",
-    "detector_sigma_a_keV": 1.0,
-    "detector_sigma_b": 0.0,
-    "detector_tail_fraction": 0.0,
-    "detector_tau_keV": 5.0,
-    "detector_fano_factor": 0.12,
-    "detector_n_channels": 4096,
-    "detector_channel_energy_range": [0.0, 0.35],
 }