[mvilacad]

Closes #573
Closes #574

Stacked on #578 (until it lands, the diff against main includes its commits).

What ships

• Viewport-complete UX concept (/docs/concepts/viewport/): vocabulary, 11 sourced claims with scope, the contract, library mapping (--ca-vh, --ca-vh-min, --ca-safe-*, keyboardAware, StickyActions, MobilePane), anti-patterns, checklist, AI prompt, Limits, References. Three demos (#988-#990): a viewport simulator (device shape, bars, safe areas, keyboard) backed by a unit-tested model, a hero that is a floor, and a form with the keyboard open.
• Three more concepts (docs: concepts batch 2: three more concepts and the 20-concept backlog #574): motion that respects people, targets and focus, appeal is not usability. The 20-concept backlog is on the concepts index.
• MCP list_concepts and get_concept.
• llms-full concept digest is the lead, the contract and the prompt; checklist, anti-patterns and evidence stay in /llms/concepts/<id>.txt.

Fail before, pass after

• axe on the four concept pages: critical label (GFM task lists render unlabelled checkboxes). Checklists are plain lists, guarded by a test; axe passes on all routes in dark, light and forced colors.
• llms-budget: llms-full was 355,007 B over the 350,000 cap with four concepts; the digest change brings it to 347,811 B.
• Demo catalog check failed on spec lines not found verbatim in the sources; sidecars fixed, manifest --check ok.
• a11y_no_noninteractive_tabindex on the scrollable hero frame; fixed with the repo's pattern.

Verified

viewport-model, mcp-concepts, evidence-rules, llms-full, llms-budget, ai-entry tests; site build; layout guard (37 passed) and axe on the concept routes; pre-push validate: ok.

Left open

Writing the other 16 concepts of the backlog; each needs its sources opened first.