v0.29.0

No schema change: x-schema-version stays 1.16, and every manifest and
lesson valid under 0.28.0 is valid under 0.29.0. What moves are two author
lints, a new package subpath and a new data file.

W-DOMAIN-UNKNOWN also on a lesson's own domain (engine#183)

The lint checked only the domain of a manifest's set entries. A lesson may
carry its own domain (absent or null inherits the set's), and the real
case sat exactly there: two exported book sets with "domain": "imported" on
every lesson. validateLesson now reports the same warning at /domain,
through the helper the manifest check uses, so both say the same thing. No
value is special-cased: the engine does not know the origin markers of
individual consumers. Measured over the ten content repos before shipping:
no lesson draws it today.

One W-HINT-LENGTH rule (engine#186)

The hint-length rule existed twice, as this warning and as an error in the
content template's validator, and the two disagreed. The engine's version
matched a number word and a length noun anywhere in the hint, without word
boundaries: over the ten content repos it reported 44 warnings, all false
("Achte" read as "acht", "bestimmten" as "ten", "Fragezeichen" and
"characteristic" as length nouns). It also missed the article and
single-character forms ("mit einem Buchstaben", "ein einzelnes Zeichen", "a
single character"), eleven and twelve, the "-buchstabig" adjectives, and
every blank hint.

The rule is now a count directly before a length noun, with Unicode-aware
boundaries, plus the reversed colon form ("Buchstaben: 4", plural counts only)
and the "-buchstabig" adjectives. It checks the exercise hint and every
blanks[].hint, each at its own path; card hints stay unchecked. It stays a
warning. Measured with the build over the ten content repos: 44 warnings
before, 0 after.

learn-content-engine/rules: the semantic rules without ajv or node:fs (engine#191)

A browser consumer that has already shape-checked its input can now call the
engine's rules instead of re-implementing them: validateLessonRules,
validateManifestRules, isSlugId (the schema's $defs/SlugId),
SLUG_ID_PATTERN and SLUG_ID_MAX_LENGTH. validateLesson and
validateManifest check the shape with ajv and then call exactly these
functions, so the two cannot disagree; a test pins the parity on every
conformance fixture. The entry imports neither ajv nor node:*, and a test
walks its module graph to keep it that way. Measured with esbuild (minified,
browser): the entry is 21,590 bytes (8,320 gzip); validateLesson alone is
144,637 bytes, 114,312 of them ajv, and it reads the schema with node:fs.

Internally, src/validate.ts is cut along its three layers: issues.ts
(issue types and helpers), rules.ts (semantic rules and author lints),
validate.ts (the structural layer). The public API of the package root is
unchanged.

schema/grading-presets.json: a sourced catalog of grading scales

Exported as learn-content-engine/schema/grading-presets.json: grading scales
an author can copy into a set's evaluation block instead of typing a table.
23 presets (2 base forms, 12 class A following a primary source, 9 class B
naming their convention), 31 templates that carry a scale's grades and pass
mark but no thresholds, and 3 scales that cannot be expressed as percentages
(ECTS, GCSE/A-level, the German state law examination), each with the reason.
Every entry names its sources. A preset is copied: a later correction does
not reach sets that already carry it. label is the grade as written,
label_native the official word. Where an official lower bound is not a whole
percent, the row carries the whole percent below it and lists the official
value in rounded_rows. Tests hold every preset to validateManifest without
an error or a warning. See Grading presets.

Docs

• docs/architecture.md: "Rule ownership: which layer owns which rule". The
  engine owns every rule about the content itself; the content template's
  tooling owns what the engine cannot see; the app owns render-time decisions.
  With the known violations, as of 2026-09-24, and where each is tracked.
• The grading presets explain why flooring a threshold is exact on a point
  scale, and that it maps the national scale, not a consumer's run (a run on
  its own grid can land between two reachable scale points).
• Every guide that pointed at src/validate.ts for adding a rule now points at
  src/rules.ts; README lists the two subpath entries.
• CONTRIBUTING.md: a release now brings every doc up to date first, with the
  audit recorded in the release PR.
• The project language is English: code, comments, docs, commit messages, PR
  and issue texts (.claude/rules/coding-standards.md); tdd.md is
  translated without a rule change.