feat(quality): a bridge lesson has no exercise-type minimum either; schema 1.18 (#185, #182)

[astrapi69]

Summary

• purpose: "bridge" now lifts the exercise-type minimum too. Owner decision of 2026-09-25, following the recommendation from Quality minimums in the engine, keyed to what a lesson is for #185: a lesson without an assessment intent needs neither count nor variety. Before this, a bridge lesson of theory alone, or with exercises of one type, fell short on E-QUALITY-TYPES. The theory minimum and the per-exercise minimums still apply. The change only relaxes, so nothing that passed before falls short now.
• Schema 1.18 changes no field.
  • The description of purpose says what bridge lifts.
  • Closes Schema 1.16: two descriptions lost a line break in the em dash rewrite #182: the root Lesson and InlineExample descriptions get back the line breaks that the em dash rewrite of 1.16 swallowed. Schema 1.16: two descriptions lost a line break in the em dash rewrite #182 was meant to travel with the next schema change and missed 1.17; this is that change.
  • The manifest schema, quality-rules.json and grading-presets.json move their version in lockstep.
  • Regenerated: types, the schema module, baselines and diagrams.

Test plan

• RED first. Three tests failed before the change:
  • a bridge of theory alone passes;
  • a bridge with one exercise type passes;
  • a bridge keeps the theory and per-exercise minimums.
• make release-check, make prose-check and npm run docs:api:check pass.

Docs:

• lesson-format.md: minimum table, bridge bullet, catalog row, meta row, the version-counter note.
• concepts.md gains the 1.18 note.
• Version claims 1.17 → 1.18 in README, the comparative analysis and six blog files (en and de). The bridge notes in the schema-first blog are updated in both languages.
• CHANGELOG.

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