Which scheme is embeddable and lightweight?

And what does lightweight mean? Does it mean low memory footprint or does it mean few-lines-of-code-to-introduce or does it mean zero-dependencies?

https://en.wikipedia.org/wiki/SIOD is lighter weight than almost anything. Way smaller than Lua.

Also look at Hedgehog Lisp. The bytecode compiler (runs on a PC) is separate from the interpreter, i.e. there is no REPL. But it means that the interpreter is only about 20KB of code. It's quite practical. It's not Scheme but rather is a functional Lisp (immutable data including AVL trees as the main lookup structure) and it is tail recursive. https://github.com/sbp/hedgehog