v0.42.2

x-schema-version moves with every schema change, and only then (engine#279)

0.42.1 changed a schema description and kept x-schema-version at 1.19,
although the counter bumps on every change to the schema definition,
description-only edits included (docs/lesson-format.md, since #139). Two
different contents were published as 1.19, and a consumer that compares the
number to see whether the definition moved saw nothing.

• 1.20. Both schema files move to 1.20, in lockstep as always; the
  content is 0.42.1's. quality-rules.json and grading-presets.json carry
  the new $schema-version.
• A ledger gate, both directions. src/__fixtures__/schema-baseline/schema-version-ledger.json
  maps each version to the digest of the schema set's content (both files,
  $id and x-schema-version removed). src/schema-version-ledger.test.ts,
  part of make release-check, fails when:
  • the content changes under an unchanged number;
  • a number rises without a change;
  • the two files are out of lockstep;
  • two rows share a digest.
    Seeded negative probes cover each case, on a ledger that is consistent by
    construction. Before the bump the test failed on this repository with "the
    schema content changed but x-schema-version is still 1.19".
• History. A check over every release tag found content that changed
  under an unchanged number seven times. Five predate the rule: 0.7.0, 0.8.1
  and 0.8.2, 0.14.0, 0.20.0. Two came after it: 0.24.1 (1.14) and 0.42.1
  (1.19). A number that rose without a change never occurred.
• Docs name no version where none is needed. README, concepts.md,
  architecture.md, comparative-analysis.md and the three blog posts in
  both languages no longer state the current schema or package version; the
  schema file states it. A diagram in comparative-analysis.md still said
  "schema 1.15" four versions later, out of reach of the version-claims gate.
  The manifest schema_version default in lesson-format.md stays: authors
  need it, and it keeps the version-claims gate from scanning blind.