feat(docs): link the changelog in and deep-link each board fix to its build

The generated changelog was an orphan: no page linked to it, and nothing linked from it, so
"generated and maintained" still meant "nobody lands here".

- guide.html + help.html footers and the board header now link the changelog (and the board, from
  the app side). All three verified live: HTTP 200, 1 link each.
- Every closed board card gains "fixed in <sha> · v4.6.XXX" where the version is a link to
  changelog.html#v4-6-XXX. The anchor rule is the changelog renderer's own
  (non-alphanumerics -> "-", lowercased), not a second guess at it.
- The page now actually jumps to a hash. Previously the list is built by JS after load, so the
  browser's own anchor scroll ran against an empty list and every deep link landed at the top.
  jumpToHash() runs after render and on hashchange.
- A build can legitimately be missing: v4.6.83 fixed MAINT-002 but never reached a commit (the
  version was superseded before anything was committed), so #v4-6-83 has no entry. Rather than
  dropping the reader at the top, the page lands on v4.6.84 and says why in a hint line.

Verification, live:
- 22 cards carry a changelog link; board header shows "Release notes: changelog.html".
- #v4-6-107 -> targetExists true, :target highlight true, top 0, scrolled into view, no note.
- #v4-6-83  -> scrollY 5235, note: "#v4-6-83 has no commit of its own — the version was
  superseded before it reached a commit — so this is v4.6.84, the next build that did."
- renderer JS and board JS both pass node --check on the extracted <script> bodies.

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