feat(ui): add the Tools menu that reaches the diagnostic and field-test pages

[clonea1]

Several shipped pages are reachable only by typing their URL, because the main
nav has no entry for them. This adds a Tools dropdown grouping them under Field
tests, Diagnostics and Views.

This PR is wiring only and MUST merge last. It adds no pages. Eight of the
eleven links point at pages that arrive with other PRs, so merging this first
would put dead entries in the nav. Three are already present here
(viz.html, observatory.html, pose-fusion.html) and would work today.

Deliberately kept as one small follow-up rather than splitting a menu line into
each page's PR: a nav menu is a single coherent thing, and assembling it three
lines at a time across eight PRs produces eight conflicts in one file for no
benefit. Reviewing it once, last, against the pages that actually landed is
both safer and less work.

The dropdown is plain markup plus a small inline handler -- no framework, no
build step, no dependency. It closes on outside click (including touch, where
there is no pointer to move away) and on Escape, and carries the
aria-haspopup / aria-expanded / role=menu wiring so it is reachable without a
mouse.

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Merge order

This is the last piece. Each link below arrives with another PR; merging this
before them leaves dead entries in the nav.

menu entry	page comes from
Walk Test Runner, Tile Survey Runner, Room Marker, Tapper	#1825 ui-ground-truth-capture
Mesh Matrix, RF Distance Map	pending — mesh.html / mesh3d.html not yet submitted
Illuminators	#1833 server-emitter-triage
World 3D	#1834 ui-world3d
3D Visualization, Observatory, Pose Fusion	already in main

Happy to hold this as a draft until the others land, or to trim it to only the
three entries that work today and follow up — whichever you prefer. Flagging it
now mainly so the pages do not merge and then sit unreachable.