Add a browser-only attrition calculator on GitHub Pages

[alihdrndm]

What

• apps/calculator: a static, browser-only attrition calculator built on @alihdrndm/blockpace-core. It shows the cumulative and per-night bases side by side, with a night-by-night breakdown. No server, no API key, and nothing typed leaves the browser.
• .github/workflows/pages.yml: builds, tests and publishes it to GitHub Pages on pushes to main that touch the calculator or core. Actions are SHA-pinned; only the deploy job gets pages: write and id-token: write.
• README: calculator link and a demo GIF of the dashboard flow (On track → snapshot → At risk → webhooks delivered).
• CHANGELOG and DECISIONS entries.

Checks

• 5 calculator unit tests (worked example 2 gives $975.60 cumulative and $1,528.80 per night).
• Lint and typecheck clean; build is about 31 KB gzip.
• Checked in a browser at desktop and mobile widths.