v0.31.0

Schema 1.17: one additive lesson field, purpose. Every manifest and lesson
valid under 0.30.0 is valid under 0.31.0; validateLesson reports nothing new.
The new check is a separate call a consumer opts into.

Quality minimums in the engine, keyed to what a lesson is for (engine#185)

validateLessonQuality(lesson) checks a shape-valid lesson against the quality
minimums of schema/quality-rules.json (exported as QUALITY_MINIMUMS): at
least five exercises of at least two types, a theory step, two accepted answers
per free_text, three pairs per matching. Shortfalls come back as errors
with five new ids, E-QUALITY-EXERCISES, E-QUALITY-TYPES, E-QUALITY-THEORY,
E-QUALITY-FREETEXT-ACCEPTS and E-QUALITY-MATCHING-PAIRS, each with count
and min in its params. It is also on the learn-content-engine/rules entry.
See quality minimums.

The minimums are a publication threshold, not validity: validateLesson
never reports them, so a consumer that generates short lessons (the reference
app's adaptive lessons) still accepts its own output, and each consumer gives
a shortfall the weight of its gate.

Schema 1.17 adds the optional lesson field purpose: practice (the default,
every minimum), bridge (no exercise minimum) and quiz (no exercise-type
minimum). Content without it validates unchanged; the manifest schema moves
its x-schema-version in lockstep and changes nothing else.

Why: the minimums existed in three versions (the content template's gate,
alc-books' copy, the app's share check) with different exemptions and a
different count for matching with from_cards, so a set could pass its
repository gate and fail to be shared. The one version here replaces the
template's multiple-choice exemption and alc-books' bridge lessons recognised
by their id with the declared purpose, and counts the pairs from_cards
derives. The distractor requirement the three versions put on free_text and
picture_choice has no counterpart. Measured over the 631 lessons of the ten
content repositories on 2026-09-25: 8 fall short today (seven bridge lessons
in alc-books, one quiz in alc-traffic-knowledge), none once those eight
declare their purpose.