Add example fpr callback to notebook

Author: FBumann

docs/notebooks/04-operational-constraints.ipynb

@@ -417,6 +417,84 @@
    "cell_type": "markdown",
    "id": "28",
    "metadata": {},
+   "source": [
+    "## Custom Constraints\n",
+    "\n",
+    "Sometimes you need constraints beyond what's built into the components. The `before_solve` callback lets you add custom constraints directly to the optimization model.\n",
+    "\n",
+    "### Example: Limit Main Boiler Startups\n",
+    "\n",
+    "Let's limit the main boiler to a maximum of 3 startups:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "29",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# Build a fresh system with status tracking on both boilers\n",
+    "fs_custom = flow_system.copy()\n",
+    "fs_custom.reset()\n",
+    "\n",
+    "\n",
+    "def limit_main_boiler_startups(fs):\n",
+    "    \"\"\"Custom constraint: Main boiler can start at most 3 times.\"\"\"\n",
+    "    model = fs.model\n",
+    "    main_startups = model.variables['MainBoiler|startup']\n",
+    "    model.add_constraints(main_startups.sum() <= 3, name='max_main_startups')\n",
+    "\n",
+    "\n",
+    "fs_custom.optimize(fx.solvers.HighsSolver(mip_gap=0.01), before_solve=limit_main_boiler_startups);"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "30",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# Verify: Main boiler startups are limited\n",
+    "main_startups = int(fs_custom.solution['MainBoiler|startup'].sum().item())\n",
+    "print(f'MainBoiler startups: {main_startups} (limit: 3)')\n",
+    "\n",
+    "# Compare with unconstrained (which had more startups)\n",
+    "print(f'Original system startups: {total_startups}')\n",
+    "\n",
+    "# Show startup events\n",
+    "startup_ds = xr.Dataset({'MainBoiler startup': fs_custom.solution['MainBoiler|startup']})\n",
+    "startup_ds.plotly.line(x='time', title='Main Boiler Startup Events', height=250)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "31",
+   "metadata": {},
+   "source": [
+    "### Finding Available Variables\n",
+    "\n",
+    "To discover what variables you can use in custom constraints, inspect the model after building:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "32",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# List all variables in the model\n",
+    "print('Available variables:')\n",
+    "for name in fs_custom.model.variables:\n",
+    "    print(f'  {name}')"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "33",
+   "metadata": {},
    "source": [
     "## Key Concepts\n",
     "\n",