Relocate codegen.rs + codegen_targets.rs out of openpencil-shell-core
into a dedicated op-codegen crate. The code generators only depend on
jian-ops-schema + op-editor-core, so they form a clean leaf crate.
codegen.rs becomes the crate's lib.rs; crate::codegen:: paths in
codegen_targets.rs collapse to crate::. Pure relocation, no behaviour
change; tests move with their code.
