The op-* crate reorg never ran `cargo clippy -- -D warnings`, so the CI
lint gate failed. Fix every violation surgically: real fixes for
mechanical lints (needless_range_loop, derivable_impls, ptr_arg,
needless_borrow, doc_lazy_continuation, unused_imports, manual_find,
collapsible_match, never_loop, dead_code, complex types via type
aliases) and scoped `#[allow]` for intrusive ones (too_many_arguments
on paint helpers, result_large_err where ToolOutcome / PenNode payloads
are deliberately the Err type).
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.
