docs: add ADR-0026 — complex WHERE expressions

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.

docs/adr/README.md

@@ -31,3 +31,4 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0023 — Unified declarative grammar tree](0023-unified-grammar-tree.md) — direction (superseded for execution detail by ADR-0024)
 - [ADR-0024 — Unified grammar tree: execution plan](0024-unified-grammar-tree-execution-plan.md) — **Accepted**, the executable spec — implemented (Phases A–F; Phase F shipped "minimal", `parser.rs` retained as the router — see the ADR's Phase F implementation note)
 - [ADR-0025 — Indexes](0025-indexes.md) — **Accepted**, `add index` / `drop index`, persistence, rebuild-table preservation, and items-list display (`C3` index portion + `S2`)
+- [ADR-0026 — Complex WHERE expressions](0026-complex-where-expressions.md) — **Accepted**, stratified recursive expression grammar (`AND`/`OR`/`NOT`, comparisons, `LIKE`, `IS NULL`, `IN`, `BETWEEN`) for `update` / `delete` / `show data` filters; `show data` gains `where` + `limit`; adds the `Subgrammar` node and a recursive `Expr` AST (`C5a`)