Library exports a server JLink component

[romain-pm]

Part of #749. The component over getLinkProps. Server-only.

<JLink node={props["j:linknode"]} className={classes.cta}>{label}</JLink>

• Renders a bare <a>: no class, no wrapper, no styling opinion — the same stance as <JImage>.
• JLinkProps is a discriminated union over node | content | href, so "exactly one of" is a type error rather than prose, and the href branch requires children or aria-label. An anchor with no accessible name is a WCAG 2.4.4 failure, and it is the real analogue of JImage's required alt.
• whenUnresolved: "children" (default) renders the children unwrapped, "none" renders nothing. Not named fallback — JImage's fallback is a static-asset URL, and the two components are presented as a matched pair.
• Never an <a> without href. That is what luxe's CTA produces today, and its test asserts the resulting <a>Broken</a>.
• Emits aria-current="page" from state.isCurrent. No LinkState key reaches the DOM — assert this in a unit test, so a field added later cannot leak.
• Arbitrary children, so <JLink node={target}><JImage node={cover} alt={alt} /></JLink> works — core's jnt:imageReferenceLink family, which Digitall ships instances of.
• All other anchor attributes pass through untouched (onClick, aria-label, hreflang, lang, download).
• Importing it from a client bundle already fails at build time (vite-plugin/src/index.ts:214-221, :316-321). Make sure the error names JLink's server-only status and points at getLinkProps + <a {...anchor}>.
• Naming: JLink, for the same reason <JImage> is not <Image>.

First pass in #751.