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/cli.py

@@ -372,8 +372,6 @@ def _print_dry_run_output(config, source_type: str) -> None:
     else:
         print("Mode: single-branch")
 
-    # Detector response is a Phase 4 feature — not yet implemented
-    print("Detector:   not configured (Phase 4)")
     print("========================================")