cross: list: document why we use the flat impl

crates/cross/src/compound/list/flat.rs

@@ -1,3 +1,9 @@
+//! list implementation where indexing is done non-recursively.
+//! this puts a hard limit on the size of a list which can still be indexed (based on macro impl)
+//! but works around a limitation in rust-gpu's spirv codegen which otherwise makes lists
+//! containing ZSTs break the compiler.
+//! this ZST bug should be fixed in later rust-gpu revisions. see: https://github.com/EmbarkStudios/rust-gpu/commit/03f89e8ba6f236218b3c5f9b18fe03c25a4d6a5c
+
 use crate::compound::list::{Indexable, Meta};
 use crate::compound::peano::{P0, Peano, PNext};