fix(md): give islands the page frontmatter when they hydrate

[Brooooooklyn]

Follow-up to #475.

Since #475, an island on a Markdown page sees the page frontmatter through useFrontmatter() during server rendering, but {} in the browser. React, Svelte and Vue then report a hydration mismatch and re-render the island with {}. Vue islands had the same gap before #475.

server:   island renders useFrontmatter().title → "Hello"
browser:  island hydrates as its own root, no page context → {} → mismatch, shows "undefined"

Behavior

useFrontmatter() now returns the page's frontmatter everywhere on a Markdown page: layouts, the page, and islands, both on the server and in the browser.

• A page that renders at least one island and has frontmatter includes it once: <script type="application/json" id="__void_frontmatter">. Pages without islands or without frontmatter send nothing new.
• initIslands reads it and passes it to each adapter's island hydration, which provides it the same way the server does: React and Solid FrontmatterContext, the Svelte hydrate() context, Vue provide.
• React and Solid load @void/react / @void/solid for this only when the page has frontmatter.
• The Markdown guide now says islands can read it, and that frontmatter on pages with islands is visible in the HTML.

Why send it, instead of giving islands {}

Astro, îles and Fresh islands get only their props. Here, though, useFrontmatter() already works in layouts and pages. If islands silently got {}, users would fall into the same trap as #451. Frontmatter is already public through @void/md/pages, and the cost is one small JSON script on pages with islands.

Tests

• A browser test per adapter (md-layout-frontmatter.test.ts for React, Solid and Svelte, and the new md-island-frontmatter.test.ts for Vue) uses a shared helper, test/island-page-browser.ts. An island layout loads an island that prints useFrontmatter().title. Each test checks the JSON script is in the server HTML, then that the island still shows the title after hydration with no hydration warning.
• Mutation checks: removing the browser-side provider for any one adapter makes its test show undefined. Removing the server script fails all four.
• The fix(md): let layouts read the page frontmatter with React, Solid and Svelte #475 layout tests now also check that a page without islands gets no frontmatter script.
• islandFrontmatterScript escapes </script> inside frontmatter values (deferred-xss.test.ts).
• Run locally against the installed Chrome, because the Playwright Chromium download is blocked here. The existing react-island-hydration test still passes. Unit tests for void, md and the four adapters, lint, format and typecheck all pass.

🤖 Generated with Claude Code

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-18T09:48:08.223696Z	2bc7aa8	Manual request

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Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: 2bc7aa8925

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