Add note explaining whole-pointer order requirement

Let's add an admonition explaining that bytes with provenance must
form a complete pointer in the correct order.

Author: traviscross

src/const_eval.md

@@ -260,6 +260,11 @@ const C: Pair = unsafe {
 };
 ```
 
+> [!NOTE]
+> The bytes with provenance must form a complete pointer in the correct order. In the example above, the pointer is written at offset 20, but a pointer requires 8 bytes (assuming an 8-byte pointer). Four of those bytes fit in the `y` field; the rest extend into the padding at offset 24. When the fields are initialized, the `y` bytes get overwritten, leaving only a partial pointer (4 bytes) in the padding. These 4 bytes have provenance but don't form a complete pointer, causing compilation to fail.
+>
+> This restriction ensures that any bytes with provenance in the final value represent complete pointers. Binary formats such as ELF cannot represent pointer fragments, so the compiler cannot emit them in the final binary.
+
 > [!NOTE]
 > Manually initializing (e.g., zeroing) the padding bytes ensures the final value is accepted:
 >