docs(readme): live receipts — status demo gif, metrics ledger, Built-with-Roll stats

[seanyao]

What

• Hero demo gif: animated roll status capture — real output from this repository (1,137 stories merged, health numbers up front)
• Built with Roll section (EN + ZH): the repository's own development numbers, read from git history and the delivery ledger — 116 days, 3,478 commits, 1,506 PRs, +686k/−215k lines, ~324k lines TS/JS, median daily commit span 20.3h, ~163M/38M tokens ≈ $2.8k (estimated, per roll metrics itself)
• docs/assets/readme/: demo gif, status + metrics renders, and a 1280×640 social-preview image (to be uploaded in repo Settings)
• scripts/render-readme-assets.py: reproducible generator for all of the above

Notes

• The roll metrics view currently ships zh-only; the README says so rather than hiding it.
• Token/cost figures are the ledger's own estimate and are labeled as such.
• Docs-only change; no behavior touched.