Split site navigation from in-page navigation

docs/index.html's bar carried eighteen links: six anchors into its own
sections, eleven destinations, and the CTA. At 1280 that wraps to two
rows, and the two kinds of link behave completely differently when
clicked. Every other page's bar held nine, so the home page was also the
one page whose chrome looked different from the rest of the site.

The bar is now the same eight destinations everywhere: the primary path
through the docs, one row at every width. Its six section anchors are a
jump row under the hero, where in-page links point at in-page targets.

demo.html ("Try the search") and chat.html ("How chat works") were
reachable only from index.html's bar — from any other page there was no
way to reach them at all. Both are now in the bar and in the footer
index, which is on every page.

Roadmap, Design and Carousel move out of the bar and stay in the footer
index. A bar that lists every page is a site index, and there is already
one of those at the bottom of every page; three meta pages were crowding
out the demo. Any of them goes back in one line.

Verified: a11y_check (10 pages, 24 contrast pairs), check_anchors (every
local link and anchor resolves), build_roadmap --check,
build_command_reference --check, and pytest tests/unit tests/functional.
The two TestServe failures are the sandbox refusing socket.bind(), not
this change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
