feat: link compared frameworks from the /compare pages

[vivek7405]

Closes #856

What

Adds one outbound link per comparison to the competitor's official site, so the /compare pages cite what they compare (a mild positive SEO signal + credibility/E-E-A-T, and it reads as honest rather than evasive).

How

• New link frontmatter field on each compare/*.md, threaded through modules/compare/types.ts and both queries.
• The [slug] page renders the "WebJs vs " header eyebrow as the outbound link (one prominent link per page, above the fold, no body clutter). Index cards are left un-linked to avoid a nested anchor (the whole card is already a link to /compare/<slug>).
• Links: nextjs.org, lit.dev, remix.run, astro.build, rubyonrails.org. The Remix 3 page also links its "beta preview" mention to the Remix 3 beta post.
• All outbound links are follow (genuine editorial citations, not paid/untrusted), open in a new tab, and carry rel="noopener noreferrer".
• The shared blog/compare markdown renderer (render-post.ts) now opens absolute-URL links in a new tab; internal / links still navigate in place. This is why the Remix inline beta link opens in a new tab, and it is a small improvement for blog external links too.

Verification

• webjs check: all checks pass.
• Booted the site: every /compare/<slug> renders its outbound eyebrow link with target="_blank" rel="noopener noreferrer"; the Remix beta-post inline link is present and new-tab; index cards link only to /compare/<slug> (no nested anchors); the blog still boots and its internal links stay same-tab (0 mis-targeted).

Docs / surfaces

• N/A for framework docs: this is website marketing content only, no packages/* public surface changed. The link field is a website-local frontmatter convention.

[vivek7405]

First pass over the outbound-link change. The mechanics are sound (link threaded through the frontmatter, types, and both queries; the eyebrow hole is a correctly-unquoted webjs attribute hole; index cards stay plain text so there are no nested anchors), but three things needed fixing before this is ready:

1. Both new-tab surfaces skipped the site's own a11y convention. The chrome appends a visually-hidden "(opens in a new tab)" cue (lib/links.ts NEW_TAB) on every external link, and there is a layout test asserting it. The compare eyebrow and the renderer's external links now append the same cue.
2. No test shipped for either new behavior. Added SSR/unit coverage for the renderer's external-vs-internal branching (with the internal counterfactual) and the eyebrow link.
3. The renderer interpolates the markdown URL into a double-quoted href without escaping quotes. Pre-existing and the source is trusted repo markdown, but this PR widens that surface, so a " in a URL is now percent-escaped so it cannot break out of the attribute.

All three are fixed in the follow-up commit; tests and check are green.

[vivek7405]

Second pass, focused on the shared renderer's link edge cases and the test. Two real gaps in the external-vs-internal branch, both fixed; one pre-existing renderer limitation filed as a follow-up; the tests hold up as real counterfactuals.

• The external check was case-sensitive and missed protocol-relative URLs, so HTTPS://... and //host were treated as in-place internal navs (no new tab, no rel, no cue). Broadened the check to /^(https?:)?\/\//i and added an uppercase-scheme test case.
• The link regex truncates a URL at the first ) (so a Wikipedia-style ..._(language) link loses its tail). Pre-existing, not introduced here, and out of scope for this PR, so filed as #860.
• Confirmed the internal-link assertion is a genuine counterfactual and the compare-page test is deterministic (getComparison resolves off import.meta.url, not cwd).

Documented the new-tab behavior in the renderer header. Tests and check green.

[vivek7405]

Third pass, cross-file consistency and completeness. link is threaded identically through the type and both queries; the [slug] eyebrow ternary is well-formed (empty string falls to plain text, the link branch nests a single anchor with the NEW_TAB cue); the index cards correctly omit the outbound link to avoid a nested anchor; no invariant-11 punctuation in the new prose; no sitemap/metadata surface needs the link. Nothing left to fix.

[vivek7405]

Reviewed the visible-link change (abae1b3). The eyebrow is back to plain text and the outbound link is now an explicit, underlined "Visit the site" link under the tagline with an aria-hidden arrow and the screen-reader cue. Template is well-formed (href is a normal unquoted attribute hole), no nested anchors, no dead code, and the test's label assertion pins the visible link so a revert to hover-only would fail it. Nothing to fix.