Add progress.asc task checklist and a blog progress page - #9

Summary

• Adds progress.asc at the repo root: a plain AsciiDoc checklist laying out every third-edition task, derived from REVISION_PLAN.md. It is meant to be edited by hand — flip [ ] to [x] to mark a task done, add tasks or sections as they come up. Standard checklist syntax, so it also renders as checkboxes on GitHub.
• Adds a progress page to the site at /blog/progress/ that parses progress.asc at build time (site/src/lib/progress.ts) and renders an overall progress bar plus per-section cards with bars, counts, and check-marked task lists.
• Adds a summary card at the top of the blog index linking to the progress page, showing the current overall bar and count.
• Adds a blog post, "A progress page for the third edition", introducing the page and explaining how progress.asc works as the hand-edited source of truth.

Notes

• The parser's contract is minimal: == Section headings and * [ ] / * [x] items (nested ** items indent). Everything else in the file is ignored, so prose notes in progress.asc are safe.
• Verified progress.asc converts cleanly with Asciidoctor, and npm run build in site/ generates /blog/progress/ and the new post (currently 5 of 75 tasks, 7%).

Demo