0.28.0 - W-CLOZE-NO-CARRIER and schema 1.16

Two changes travel together so consumers re-pin once, not twice.

W-CLOZE-NO-CARRIER (engine#178)

A cloze whose sentence is nothing but its blanks has no gap text to read around: the question sits in the prompt and the blank teaches nothing. That exercise is a question with an answer, and a native type says so directly - multiple_choice in select mode, free_text in type mode. Both exist (multiple_choice since schema v1.6), so the cloze spelling is a workaround that outlived its reason.

A warning, not an error: the content is valid and the shape can be deliberate. multiselect is exempt by design - its sentence IS the question and carries no markers.

Measured before shipping: 195 of 1406 cloze exercises across the content repos (190 select, 5 type), concentrated in three repositories, and nothing else flagged. It also caught a real defect in a set being authored at the time, before that set was merged.

Schema 1.16 (engine#180)

The authored schema files carried 22 em dashes in their field descriptions, and every consumer inherited them: both files are mirrored byte for byte into ten content repos and into the reference app, where a local fix would turn that repo's drift gate red. Fixed at the source.

No field moved. x-schema-version goes 1.15 to 1.16 in both files because it counts edits to the schema FILE, description-only ones included, while the manifest's schema_version default stays at 1.7.

Upgrading

Content repos: bump schema/engine-version.txt to 0.28.0 and refresh the mirror (python3 scripts/check_schema_drift.py --update). Nothing in existing content becomes invalid; the new lint reports, it does not block.

Full notes: CHANGELOG.md.