chore(rpkg): drop stale module-level comment from lib.rs

[CGMossa]

AI-written details

Summary

• Drops two //! lines from dvs-rpkg/src/rust/lib.rs that described the crate's return shape as "R DataFrames (and Lists) via miniextendr's AsSerializeRow / ColumnarDataFrame converters". The wording predates the current API and the parenthetical "(and Lists)" is now misleading — most dvs_* functions return ColumnarDataFrame, not List.
• Carved out of docs(spec): remaining audit edits (errors, init/add/get/status text + R signatures, audit-log JSON, threading) #149 (spec audit april) as part of the split into focused PRs. Pure mechanical hitchhiker, no spec content here.

Test plan

• just rpkg-clippy -- -D warnings clean
• just rpkg-test passes

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Drafted by Claude (claude-opus-4-7). Reviewed by the author.