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).
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