feat(ua): expose low-entropy user-agent as --const-ua-* (plugin + head)

[argyleink]

What

Adds a ua global plugin and matching head constants that surface the device/browser identity JS can read but CSS can't, as five write-once custom properties:

prop	value
--const-ua-platform	macos / windows / linux / android / ios / chromeos / unknown
--const-ua-browser	chrome / edge / firefox / safari / opera / samsung / unknown
--const-ua-engine	blink / gecko / webkit / unknown
--const-ua-version	browser major version (number, 0 if unknown)
--const-ua-mobile	1 on a phone-class device, else 0

Now CSS can branch on the client with no JS and no media query:

@container style(--const-ua-engine: webkit) { .sticky-fix { position: -webkit-sticky; } }
@container style(--const-ua-platform: ios)  { .cta { padding-block: 0.9rem; } }
.badge { opacity: calc((var(--const-ua-version) - 100) / 20); }

How

• src/core/ua.ts — shared reader. Prefers structured UA Client Hints (navigator.userAgentData, Chromium), falls back to a best-effort userAgent-string sniff on Firefox/Safari. SSR-safe.
• src/plugins/ua.ts — global source, const cadence, typed (<custom-ident> strings, <integer> numbers). Registered in plugins/index.ts + loaders.ts so auto lazy-loads it.
• src/head.ts — writes the same five consts synchronously → FOUC-safe, pre-paint branching (dual-availability, like nav-type). Shared code split into the head and plugin chunks; core index.js stays lean.

Privacy

Low-entropy only — no getHighEntropyValues, device model, full version, or architecture. Everything here is already sent in request headers, so it adds no fingerprinting surface beyond what the server already sees. Off Chromium the string fallback is best-effort — hints, not identity.

Demos & docs

• Demosite: new ua card at the end of "Every source, at a glance" (string idents rendered via the same style-query trick as nav-type).
• Docsite: interactive /demos/ua/ page (chips light up for the detected OS / browser / engine), plus reference/plugins, getting-started, README, and llms.txt kept in sync.

Verification

• npm run typecheck clean · 110 unit tests pass (6 new for the reader) · npm run build succeeds
• Code-split correctly: UA logic in head.* and the plugin chunks, not in core index.js
• Real-Chromium DOM check: :root computes platform=macos, browser=chrome, engine=blink, version=148, mobile=0; gallery labels and docs chips highlight the right values

Version bumped 0.7.7 → 0.7.8 (additive → patch) with a CHANGELOG.md entry.

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