6e42a118a3
The C5a design: a stratified, recursive WHERE-expression grammar (AND/OR/NOT, comparisons, LIKE, IS NULL, IN, BETWEEN) for update / delete / show-data filters; show data gains optional `where` and `limit`. Adds the `Subgrammar` reference-following grammar node and a recursive `Expr` AST, built selectively for the expression fragment. - docs/adr/0026-complex-where-expressions.md — the ADR. - docs/adr/README.md — index entry. - docs/simple-mode-limitations.md — new running list of simple-mode query boundaries vs. advanced SQL, seeded from ADR-0026. - docs/requirements.md — C5a [~] -> [ ] (designed, not yet implemented); new Documentation section with DOC1.