Opt the landing tracker into web vitals

[sarthakagrawal927]

Part of sass-maker/app-health#83 (speed program).

• Adds data-vitals and data-vitals-routes="changelog,docs,faq" to the App Health tracker tag on the Astro landing (home and shared layout). Other first segments report as /other. Inert until App Health ships vitals.js. No CSP change: the live policy already allows health.sassmaker.com scripts and https: connects (rolepatch#3 is closed).

Measurements (this Mac, SIN colo), not fixed here:

• TTFB / on new connections: 0.28 to 1.38 s (10 curls) while server-timing app;dur is 6 to 30 ms; on one keep-alive connection 0.12 to 0.30 s. The tail is outside the handler (isolate start of the OpenNext Worker; run_worker_first = true routes / through it), so caching cannot fix it.
• Desktop CLS 0.08 to 0.135 (3 runs; mobile 0.000): the hero H1/lede reflow when Geist Variable swaps in at about 2.4 s. The font is not preloaded on ui-library v0.1.11. Fix belongs with the ui-library bump (v0.1.13 adds a display-font preload, but its default face is Figtree, so this page also needs fonts.display: "geist" or a Geist preload).

Checks: pnpm quality passes locally (719 tests); landing build emits the attributes.

Drafted by Claude (not Codex: a four-line attribute change, below the call-codex threshold).

🤖 Generated with Claude Code