Bundle-size budget enforcement in CI with size-limit and PR comments

[PAMulligan]

Summary

Lock in the README's "~150KB gzipped" claim with a hard CI gate. Surface size diffs as a PR comment so reviewers can see the impact of every dependency.

Motivation

Embeddable widgets live and die by bundle size. Without a budget, dependency creep is inevitable. A failing PR check is the cheapest way to keep the team honest.

Acceptance Criteria

• size-limit configured with budgets for dist/claudius.iife.js, dist/index.mjs, and dist/claudius.css
• Initial budgets set 5% above current size; CI fails on overage
• size-limit/action posts a PR comment with size + gzip + brotli numbers
• Budget changes require an explicit commit touching .size-limit.json (documented in CONTRIBUTING)
• README badge linking to current bundle size