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/visualization.py

@@ -235,7 +235,6 @@ def _plot_spectra_view_from_df(
 ) -> Figure:
     """Plot total spectrum and per-branch spectra from CSV columns."""
     energies = df["energy_MeV"].to_numpy()
-    e_step_keV = (meta["e_step_MeV"] or 0.0) * 1000.0
 
     fig, ax = plt.subplots(figsize=(12, 7))
 
@@ -266,7 +265,7 @@ def _plot_spectra_view_from_df(
         )
 
     ax.set_xlabel(r"Electron kinetic energy $E$ [MeV]", fontsize=12)
-    ax.set_ylabel(f"Normalized Counts / {e_step_keV:.1f} keV", fontsize=12)
+    ax.set_ylabel(r"Normalized spectrum [MeV$^{-1}$]", fontsize=12)
     fig.suptitle(_spectrum_title(meta), fontsize=14, fontweight="bold")
     ax.set_yscale("log")
     ax.grid(True, alpha=0.3)
@@ -288,7 +287,6 @@ def _plot_debug_view_from_df(
 ) -> Figure:
     """Debug view (4 panels) built entirely from CSV columns."""
     energies = df["energy_MeV"].to_numpy()
-    e_step_keV = (meta["e_step_MeV"] or 0.0) * 1000.0
     active = _active_branches(meta)
 
     fig, axes = plt.subplots(4, 1, figsize=(12, 16), sharex=True)
@@ -318,7 +316,7 @@ def _plot_debug_view_from_df(
             label=f"Br. {b['index']} ({b['intensity'] * 100:.1f}%)",
         )
     ax.set_yscale("log")
-    ax.set_ylabel(f"Normalized Counts / {e_step_keV:.1f} keV", fontsize=10)
+    ax.set_ylabel(r"Normalized spectrum [MeV$^{-1}$]", fontsize=10)
     ax.set_title("Spectrum", fontsize=11)
     ax.grid(True, alpha=0.3)
     ax.legend(fontsize=8, loc="best", ncol=2)
@@ -331,6 +329,9 @@ def _plot_debug_view_from_df(
         ax.plot(energies, fermi_scaled, "r-", lw=1.5, label="Fermi", alpha=0.8)
     for color_i, b in enumerate(active):
         col = f"branch_{b['index']}_PhaseSpace"
+        if col not in df.columns:
+            # Single-branch CSVs carry the core factors unprefixed.
+            col = "PhaseSpace"
         if col not in df.columns:
             continue
         ps_vals = df[col].to_numpy()
@@ -374,6 +375,9 @@ def _plot_debug_view_from_df(
     ax = axes[3]
     for color_i, b in enumerate(active):
         col = f"branch_{b['index']}_Radiative"
+        if col not in df.columns:
+            # Single-branch CSVs carry the core factors unprefixed.
+            col = "Radiative"
         if col not in df.columns:
             continue
         ax.plot(