ci: fix mdbook edition + isolate fuzz crate from workspace

mdbook 0.4.40 (the version pinned in .github/workflows/docs.yml)
doesn't recognize edition = '2024' under [rust], failing the docs
build. Drop to '2021' for the in-book code blocks; the heuropt
crate itself stays on Rust 2024.

Adding [workspace] to the root Cargo.toml made fuzz/Cargo.toml
inherit it, but fuzz isn't in the members list — every fuzz-smoke
job failed with 'current package believes it's in a workspace when
it's not'. Add an empty [workspace] table at the top of
fuzz/Cargo.toml so cargo treats fuzz as the root of its own
workspace and stops walking up.

fuzz/Cargo.toml

@@ -1,3 +1,7 @@
+[workspace]
+# Detach from the parent heuropt workspace so cargo-fuzz can build
+# this crate independently without listing it as a workspace member.
+
 [package]
 name = "heuropt-fuzz"
 version = "0.0.0"