a5: explain generalisation mechanism in caption + ≡0 marker on zero bars

Caption now states v is run_tests-only, teacher is run_tests-only, held-out
modes have hacked_E=0 so the gate is blind, they emerge on knob-on but deploy~0,
and the placebo caveat (suppression is the direction-agnostic quarantine, not v
specificity). Bar plot tags invisible zero-height bars with ≡0.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

scripts/plot_deploy_overlay.py

@@ -79,9 +79,13 @@ def _panel(ax, by_arm, modes, arms, field, title, ylabel):
         bars = ax.bar(x + i * w, means, w, label=f"{label} (n={n_seed})", color=color,
                       yerr=yerr, capsize=2, error_kw=dict(lw=0.8, alpha=0.8))
         for b, v in zip(bars, means):
-            if not np.isnan(v):
-                ax.annotate(f"{v:.2f}", (b.get_x() + b.get_width() / 2, v), fontsize=6,
-                            ha="center", va="bottom", color=color)
+            if np.isnan(v):
+                continue
+            # a zero-height bar is invisible -- mark it "≡0" so the reader sees a
+            # finding, not a missing bar (same convention as the line plots).
+            txt = "≡0" if v < 5e-3 else f"{v:.2f}"
+            ax.annotate(txt, (b.get_x() + b.get_width() / 2, v), fontsize=6,
+                        ha="center", va="bottom", color=color)
     ax.set_xticks(x + 0.4 - w / 2)
     ax.set_xticklabels([f"{m}\n{'IN' if m == 'run_tests' else 'held-out'}" for m in modes], fontsize=8)
     ax.set_title(title, fontsize=10)