ADR-0020 + ADR-0021: tokenization layer and parse-error pedagogy (H1a)

ADR-0020 amends ADR-0001 with a two-phase parse: a lexer
producing a span-tagged token stream, then chumsky over
&[Token]. Single source of truth for keywords and punct via
a define_keywords!/define_punct! macro pattern. Parser
contract committed for I3 (queryable expected-token-set)
and I4 (lexer always succeeds, Error tokens for invalid
input). Includes an honest history note: the no-lexer shape
in dsl/parser.rs arose incrementally without ADR-level
deliberation against the known H1a/I3/I4 requirements; this
ADR corrects that.

ADR-0021 builds on ADR-0020 to close the H1a gap: a
per-command UsageEntry registry keyed off entry-keyword,
with parse errors rendered as caret + structural error +
matching usage template(s). Multi-entry families (add,
drop, show) render together. New catalog sections under
parse.usage.* (per-command grammar) and parse.token.*
(single-token vocabulary). Zero-prefix case ("frobulate
Customers") falls back to an "available commands:" framing.
Anchor-phrase compliance preserved.

docs/adr/README.md

@@ -25,3 +25,5 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0017 — Column type-change compatibility](0017-column-type-change-compatibility.md)
 - [ADR-0018 — Auto-fill contracts for `serial` and `shortid` columns](0018-auto-fill-contracts-for-serial-and-shortid.md)
 - [ADR-0019 — Friendly error layer (H1) and i18n message catalog](0019-friendly-error-layer-and-i18n.md)
+- [ADR-0020 — Tokenization layer for the DSL parser](0020-tokenization-layer-for-the-dsl-parser.md)
+- [ADR-0021 — Parser-as-source-of-truth for H1a (per-command usage in parse errors)](0021-parser-as-source-of-truth-for-h1a.md)