Add createIslandSignal for sharing reactive state across Solid islands

[matthewp]

Changes

• Adds a new createIslandSignal function exported from @astrojs/solid-js/signals that lets users create signals in .astro frontmatter and pass them to multiple client:* Solid components. All islands sharing the same signal get a single reactive instance on the client, so updating it in one island updates all others.
• Signals are detected on the server via Symbol-based branding (Solid signal getters are plain functions with no intrinsic identity, unlike Preact signals which are duck-typeable objects). The server serializes a data-solid-signals JSON attribute mapping prop names to shared signal IDs ("sg0", "sg1", etc.). On the client, a module-level sharedSignalMap creates or reuses createSignal tuples by ID.
• Supports getter props, setter props (encoded with a ! suffix on the signal ID), and signals nested inside arrays or objects.
• Signal values remain callable during SSR (unlike Preact, where .peek() scalars replace signals before render). Props are replaced with scalar values only after rendering, before Astro's serializeProps runs.

Testing

• packages/integrations/solid/test/signals.test.ts — Integration tests verifying data-solid-signals serialization: shared signals between islands get the same ID, array signals produce [id, index] tuples, object signals produce [id, key] tuples, and SSR output contains correct values.
• packages/astro/e2e/solid-signals.test.ts — E2E tests verifying signals hydrate with the correct initial value and that clicking a button in one island reactively updates a shared signal displayed in another island.

Docs

• Docs update needed to document the new @astrojs/solid-js/signals export and createIslandSignal API. Not included in this PR.

[changeset-bot]

🦋 Changeset detected

Latest commit: 350c93f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 15 packages

Name	Type
@astrojs/solid-js	Minor
@e2e/errors	Patch
@e2e/solid-circular	Patch
@e2e/solid-component	Patch
@e2e/solid-recurse	Patch
@e2e/solid-signals	Patch
@e2e/view-transitions	Patch
@test/astro-slots-nested	Patch
@test/large-array-solid	Patch
@test/postcss	Patch
@test/slots-solid	Patch
@test/solid-component	Patch
@test/astro-cloudflare-server-island-prerender-framework	Patch
@test/astro-cloudflare-with-solid-js	Patch
@test/solid-signals	Patch

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[pkg-pr-new]

npm i https://pkg.pr.new/astro@17159

npm i https://pkg.pr.new/@astrojs/solid-js@17159

commit: 350c93f

[delucis]

Is there a reason this is needed? We already document how to do this here: https://docs.astro.build/en/recipes/sharing-state-islands/

(Also seems unusual to add an API that only works for one framework when there's an agnostic approach documented.)