Shared lowered-program substrate for interpreters and copy-and-patch backends.
weavy stays format-agnostic: callers bring schema identities, parsers, and
value models. The crate provides the shared carrier for lowered programs
(flat programs, named blocks, and a small call-stack runner), plus the generic
typed-memory descriptor/op vocabulary in [mem]. Native copy-and-patch
backends can use the same program/block shape.