chore: clarify scope of packable impl detection (#24820)

Just a comment.

Author: nventuro

noir-projects/aztec-nr/aztec/src/macros/notes.nr

@@ -186,6 +186,12 @@ comptime fn generate_note_properties(s: TypeDefinition) -> Quoted {
                 let note_type_name = $note_type_name;
                 let fields_packed_len: u32 = $accumulated_offset;
                 let note_packed_len: u32 = <$typ as aztec::protocol::traits::Packable>::N;
+                // We only reject custom Packable layouts whose total packed length differs from the derived
+                // layout's. A hand-written pack() that keeps the same total length but reorders same-width fields
+                // is deliberately not caught: the selectors are computed from declared field order and would
+                // silently point at the wrong packed slot. Detecting it would require knowing whether Packable was
+                // derived or hand-written, which comptime cannot currently tell us. Custom layouts must define their
+                // PropertySelectors manually.
                 std::static_assert(
                     fields_packed_len == note_packed_len,
                     f"{note_type_name}'s auto-generated note properties assume the #[derive(Packable)] layout ({fields_packed_len} fields), but its hand-written Packable packs to {note_packed_len}. Derive Packable, or define property selectors manually for the custom layout. See https://docs.aztec.network/errors/15",