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/requirements.md

@@ -151,10 +151,12 @@ group enabled. (Earlier reference points: 1006 after ADR-0024
   writes. Bulk insert, complex WHERE expressions, and SELECT
   in advanced mode are explicitly tracked separately — see
   C5a below.)*
-- [~] **C5a** Complex WHERE expressions (AND/OR/comparison
-  operators/LIKE) for UPDATE/DELETE/show-data filtering. Tracks
-  the natural progression from DSL into real SQL fluency that
-  motivates the playground; design and ADR pending.
+- [ ] **C5a** Complex WHERE expressions (AND/OR, comparison
+  operators, LIKE, IS NULL, IN, BETWEEN) for UPDATE/DELETE/
+  show-data filtering; also adds `where` and `limit` to
+  `show data`. Tracks the natural progression from DSL into
+  real SQL fluency that motivates the playground. Designed in
+  ADR-0026; implementation pending.
 
 ## SQL handling
 
@@ -401,6 +403,17 @@ group enabled. (Earlier reference points: 1006 after ADR-0024
 - [~] **TU1** Tutorial / lesson system — design and ADR pending
   before any implementation. Out of v1 unless an ADR is written.
 
+## Documentation
+
+- [ ] **DOC1** User- and student-facing reference
+  documentation under `docs/`: the DSL command surface,
+  the type system, and the boundaries of simple mode.
+  `docs/simple-mode-limitations.md` is the first piece —
+  it doubles as student explanation and as detailed
+  reference. Distinct from in-app `help` (`H3`), the
+  interactive tutorial system (`TU1`), and the sharing
+  recipes under `E2`.
+
 ## Testing (per ADR-0008)
 
 - [ ] **TT1** Tier 1: `cargo test` + `proptest` covering