fix: notebook plot cell syntax error (newline in string literal)

The \n in ax.set_title("...\n...") was converted to a literal newline
by JSON serialization, breaking the Python syntax inside the cell.
Replaced with a single-line title.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

training/RhythmEnv_GRPO_Training.ipynb

@@ -441,7 +441,7 @@
     "    ax.axhline(0.0, color=\"k\", linewidth=0.5, linestyle=\"--\", alpha=0.5, label=\"neutral belief baseline (0.0)\")\n",
     "    ax.set_xlabel(\"Training Step\")\n",
     "    ax.set_ylabel(\"Mean belief_accuracy reward (\u22120.5 to +0.5)\")\n",
-    "    ax.set_title(\"Belief-Accuracy Reward over Training\nProof the agent learned to model the user\")\n",
+    "    ax.set_title(\"Belief-Accuracy Reward over Training (proof agent learned to model user)\")\n",
     "    ax.legend(loc=\"best\")\n",
     "    ax.grid(True, alpha=0.3)\n",
     "    plt.tight_layout()\n",