arity

Add a new built-in lint rule to the arity linter—implement it against the single-walk dispatch, register it in the one source of truth, add TDD fixtures plus the autofix-correctness (parse-clean) case, and wire up the snapshot-pinned generated docs and the two hand-maintained book indexes.

Grow arity's roxygen2 + markdown CST toward roxygen2 itself using a strict differential oracle (parser work, not formatting). roxygen2 renders #' blocks to .Rd. Use this skill to pick the next gap from the corpus, add the grammar plus projector support, lock it with a fixture + pinned tree, and ratchet the now-passing case into the allowlist. The projector at src/roxygen/project_rd.rs walks arity's CST and emits the parser-owned Rd section subtrees; the pure-Rust gate in tests/roxygen_projector.rs diffs that against a pinned <stem>.rdtree (no R, plain cargo test), allowlist-gated. The projector is a test-only faithful diagnostic: a divergence means the CST (or the encoding translation) is wrong—never patch the projector to make a case pass; fix the parser.