Add labour-burden widget: historical tax/rent/corvée burden comparison

[jbeshir]

Summary

• Stacked-bar comparison of direct tax, rent, and forced/unpaid labour burden as a share of output across 9 historical era/region buckets (Late Roman colonus through Industrial-Revolution Britain), with a toggleable modern-UK reference bar (OECD Taxing Wages + ONS/English Housing Survey).
• Every one of 27 data segments carries a named economic-history citation (Jones, Kanzaka, Blum, Goubert, Wang Yeh-chien, Thomas C. Smith, İnalcık, O'Brien, etc.) and a low/central/high uncertainty range rather than a false-precise point estimate; three thin brackets are explicitly flagged as low-confidence rather than invented.
• Sourcing is reachable in 1-2 clicks from the chart itself: per-segment keyboard-focusable detail cards, a per-bucket "Why contested?" caveat link, and a collapsible methodology + full 27-source bibliography panel — built deliberately serious/restrained in tone for a skeptical audience, per the brief.
• Stack: Preact core + a hand-authored SVG chart (d3-scale) — a disclosed, justified deviation from the repo's Observable Plot default, needed for the per-segment keyboard controls and custom uncertainty-whisker overlay. Data mode: static (committed, cited dataset — no live API exists for this data).
• Hostname: labour-burden.widgets.beshir.org. Bundle ~27KB gzip JS + ~2.2KB gzip CSS.

Pipeline / gates

• Built via a demesne-orchestrated pipeline: research (sourced dataset) → implement → validate gate (build + validate-widgets.mjs, OK) → first-paint gate (OK) → journey gate (24 cells: 4 states × 3 viewports × 2 schemes — one real defect found and fixed) → five-lens parallel UX review (4 blocking findings: whisker legibility, two dark-mode APCA Lc75 contrast failures, colour-blind-unsafe segment hues — all fixed) → a regression from the first fix round (enlarged mobile tap targets intercepting Playwright's actionability check) found via re-running the journey gate and fixed directly → clean re-validate/re-render/re-journey (24/24 cells, 0 console/page errors, 0 overflow) → final review verdict PASS.
• One repo-wide gap noted but out of scope here: scripts/journey.cjs's accessibility-tree capture (page.accessibility.snapshot()) returns null on every state, likely across all widgets — worth a separate fix.

Test plan

• npm run build + node scripts/validate-widgets.mjs pass
• First-paint gate (#widget-ready) passes
• Journey gate: 24/24 cells captured, 0 console/page errors, 0 horizontal overflow
• Five-lens UX review blocking findings fixed and re-verified
• Final review PASS
• check-widgets.yml CI on this PR
• Manual look at the render gallery before merge

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