q2r

Document markermd as q2r's primary reverse dependency (sibling ../markermd Shiny grading app) and the install-then-test check to run against it before changing q2r's public surface.

Update the docs for the review: describe the cell/table/toc subsystems and the two-phase @text rule in CLAUDE.md, swap the unused waldo Suggests for the used diffobj, add a Working with the AST README section and a NEWS.md, and narrow the vignette's tree-sitter claim.

Fix the select/rewrite/print/hierarchy review findings: register to_qmd(ts_node) and add the wrapper-class verb methods and ast_filter wrapper/list dispatch, keep shortcode keyword order and preserve ts inter-block blank lines on mutation, fix select_section level termination and the select_descendants wrapper divergence, drop the vestigial pandoc_source_info, add table and enum validators, dedup the ts walker, surface grammar-gap text and add the ts_nodes print method, and regenerate the man pages and tree-print snapshot.

Fix the helper-layer review findings across the cell, table, toc, attr, text, IO, and collection helpers: serialize NA cell options as YAML null, guard set_cell_options to executable cells, join multi-length table cells and reject zero-column or invalid-alignment tables, match Pandoc's slug algorithm, flatten citation text, harden the attr readers and setters against vector keys and NA, fix the ast_summary ordered-list crash, write QMD as exact UTF-8 bytes with a clear NUL-byte error, and add the as_df collection method and write_qmd_dir basename fallback.