Give each entry a real subpage URL (SEO)

[marsrobertson]

Right now every entry lives at /#slug on the single index.html page (by design — see README: "a standard-library Python script and one HTML file"). That's great for the zero-dependency build, but #anchor URLs don't get indexed/shared well for SEO.

Proposal

Extend build.py to also emit a real subpage per entry:

• entries/<slug>/index.html (or /<slug>/index.html at repo root) — same rendered content as the current #slug section, standalone page, own <title>/<meta description> pulled from the entry's title + "In one line".
• Keep the single index.html register/index page as-is (don't remove it — it's the browsable list).
• Cross-link: subpage has a "back to index" link; index pills link to the subpage instead of (or in addition to) the #slug anchor.
• No new dependencies — same html.escape/regex-based renderer, just called once per entry into its own file.
• .nojekyll already present, so plain directories with index.html will serve fine on GitHub Pages.

Why

• Real canonical URLs per entry = indexable, shareable, better SEO than hash fragments.
• Doesn't change the authoring workflow — still "add one markdown file to entries/, push to main."

Not urgent — filed as a TODO, not blocking anything.