Add UB warning before int_ptr.cpp example in Ch 30 / 00.pointers.ipynb

The original example dereferences `*(p + 1)` and `*(p + 2)` past a
single `int i`, which is undefined behavior. The output values shown
in the notebook happen to read adjacent stack memory on most systems,
which is what makes the demonstration useful — but students need to
know this is incidental, not a pattern to rely on.

This commit adds a markdown warning cell immediately before the code
block to make the UB explicit. The deeper refactor (switch to an
actual `int arr[3]` so the dereferences become valid) remains open
under #75 as an Option-1 improvement, since it would also require
updating the dependent cells (sample output, memory map).

Refs #75

Author: kwlee2025cpp

30.pointers-and-memory-management/00.pointers.ipynb

@@ -178,6 +178,19 @@
     "\n"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "> ⚠️ **Note on `*(p + 1)` and `*(p + 2)` below**\n",
+    ">\n",
+    "> The expressions `*(p + 1)` and `*(p + 2)` in the example *dereference* memory past the single `int i`. Computing `p + 1` as an address is permitted by the standard, but **dereferencing it when `p` does not point into an array is undefined behavior** — the C++ standard makes no guarantee about what the value will be, or whether the read will succeed at all.\n",
+    ">\n",
+    "> The output happens to show parts of adjacent stack memory on most systems (often `&p` itself, split across cells), which is what makes this a useful demonstration of stack layout *if you understand the caveat*. Real programs should not rely on this.\n",
+    ">\n",
+    "> A cleaner version of this example would use an actual array (e.g., `int arr[3] = {0, 1, 2}; int *p = arr;`), so `*(p + 1)` and `*(p + 2)` become valid accesses to `arr[1]` and `arr[2]`. See [#75](https://github.com/kangwonlee/2018pycpp/issues/75) for the deeper refactor.\n"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -1154,4 +1167,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 2
-}
+}