[aharlap]

Applies the approved spacing proposal to lib/base.css, so a question sits with its field and groups separate.

• Five rem scale steps and seven relationship tokens. Most systems (GOV.UK, USWDS, Carbon, Material, Polaris) leave 3 to 6 times more space between field groups than between a label and its field.
• One stack flow rule, plus pair rules that work whatever element wraps the pair: question to field 12px (was 0 when the pair sat in a plain div), label to field 8px (was 6), field to error 8px (was 0), group to group 24px (was 16).
• Choices and run-in summaries use 1.4 line height; legends keep their 12px margin.
• Deletes rules the shared ones now cover: three error margins (retrieval-sheet, write-distractors) and test-out's panel flow and first-panel resets. test-out keeps a zero top margin on the status that shares its title's flex row.
• AGENTS.md: conversation's role line under the person's name is a documented exception to the one-title header rule, because who the learner is talking to changes what a good reply is. The header guard test allows that one role and still fails a planted subtitle elsewhere.

Tests: computed gaps on real pattern markup at 1280 and 390; English and French at 320px with and without WCAG 1.4.12 text spacing; forced colours. The spacing spec failed on the old CSS (13 of 15).

Checks: npm test (412), typecheck, types:check, budget, demo. Browser specs run in CI.

Review: 1/6/6 0/2/4; fixes 3. Sol and Opus at 620a079 and 8fba23d; fixes in 84757e4, 825bc1f, 8fba23d, c1ebaeb and c4ad5c9. Deferred: inner px gaps in seven pattern stylesheets, narrow scene gap, test-out counter role (backlog).

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