Add a page search to the top of the left menu

[MDA2AV]

Closes #970.

The Hextra theme's search went away with Hugo, and with it the only way to find a test specification by name. The board's existing search box filters framework rows, so a query like prior-knowledge or MTU matched nothing anywhere on the site — and 124 Knowledge Base pages were reachable only by walking the tree.

What it searches

• The Knowledge Base — page titles and full body text, so MTU finds the setup page and prior-knowledge finds the H/2 validation docs.
• Every leaderboard destination — 28 profile views and the 6 composite scopes. h2c reaches both the JSON (h2c) board and its implementation page.

Results are ranked (title hit > body hit, exact title match wins outright, views float slightly above docs since they're the cheaper landing). Multi-word queries are AND. Each docs hit carries a breadcrumb and a snippet with the match highlighted. Capped at 25.

No dependency, no index file

docs.js already ships every page's rendered HTML, so the index is built from what's already in memory — 158 entries over 0.24 MB of text in ~5 ms, on the first keystroke rather than at load. Tags are stripped with a regex rather than by assigning to a detached innerHTML, which would fetch any <img> the docs grow later.

Two structural details

• buildNav() now clears #navTree, not #nav. The search box is static markup above it, so it survives the nav rebuild that every navigation triggers — otherwise typing would lose focus the moment results changed.
• On mobile, the tree is still hidden in favour of the existing <select>, but .nav is no longer display:none, so the search bar stays reachable.

Keyboard: arrows move the selection, Enter opens, Escape clears. Opening a result clears the query first, so you land on a normal menu showing where you now are. Colours come from the existing theme variables — light and dark both work.

Verified end-to-end in headless Chrome against the real data files

check	result
tree builds into its new container	3 sections ✓
fortunes → hits, tree hidden, first selected	6 hits ✓
ArrowDown moves selection	✓
click docs hit → page renders	"Fortunes (Templates)" ✓
URL stays deep-linkable	#doc=test-profiles/h1/isolated/fortunes ✓
input clears, tree returns	✓
nonsense query → empty state	✓
Escape restores	✓
1400px and 400px layouts	✓ (mobile keeps search, hides tree)

Possible follow-up, deliberately left out to keep this scoped: a / or Ctrl-K shortcut to focus the box.

🤖 Generated with Claude Code

[MDA2AV]

Closing as a duplicate — this is already in main via #1024 (squash-merged as cfbd513d). Same cause as #1028: I re-created it from the branch after the API outage cleared, without seeing that you'd already merged it.