docs: ADR-0042 — continue H1a parse-error pedagogy on the grammar tree

ADR-0020/0021 specified a chumsky-based H1a; ADR-0024 replaced chumsky
with the scannerless walker, leaving both obsolete. Mark them superseded
(kept as institutional memory) and add ADR-0042, which restates H1a
against the architecture as built.

ADR-0042 records that H1a is substantially shipped already — per-command
usage block, available-commands fallback, source-derived ident slot
labels, curated parse.custom.* near-miss messages, and schema-aware
[ERR] diagnostics — and defines the remaining work: a verified
per-command near-miss matrix (the definition of done), friendlier
literal expectation labels that add role context while keeping the
exact literal visible, and advanced-mode SQL parse parity (RETURNING
scope, CROSS JOIN ON, INSERT…SELECT count), kept distinct from
ADR-0019 §OOS-2 engine-error sanitisation.

- docs/adr/0020,0021: superseded notes + README entries
- docs/adr/0042: new ADR
- docs/adr/README.md: index upkeep (ADR-0000 rule)

docs/adr/0020-tokenization-layer-for-the-dsl-parser.md

@@ -2,7 +2,29 @@
 
 ## Status
 
-Accepted.
+**Superseded by ADR-0024** (2026-05-14). Accepted then superseded
+without being implemented.
+
+> **Superseding note (2026-06-03).** This ADR was never built. It
+> specifies a `chumsky`-over-tokens architecture — a separate lexer
+> producing `Vec<Token>`, a `define_keywords!` macro, and chumsky
+> grammar combinators consuming `&[Token]`. ADR-0024 (unified grammar
+> tree) instead adopted a **scannerless hand-rolled walker** that
+> operates directly on source bytes, and **removed chumsky from the
+> project entirely** (it is no longer a dependency). The lexer,
+> `keyword.rs`, and the token model described below do not exist.
+>
+> What this ADR got *right* survives in ADR-0024: the
+> expected-set aggregation it wanted (one branch's report no longer
+> swallowing the others) is delivered by the walker's structural
+> `expected` derivation, and the I3 (completion) / I4 (highlighting)
+> hooks it anticipated are served by the same walker. Read ADR-0024
+> for the architecture as built; this ADR remains as institutional
+> memory of the path not taken and the reasoning that led there.
+
+---
+
+*Original status (historical):* Accepted.
 
 Amends ADR-0001 (language and TUI framework) by adding a
 tokenization layer between the source string and the chumsky