feat: support iterable JSX children

[nickshiro]

Summary

ref #3054

Support finite synchronous iterables as JSX children.

• Normalize iterable children into arrays before reconciliation.
• Recursively flatten iterables nested inside arrays and other iterables.
• Preserve reactive reconciliation semantics: nodes retained between iterable updates keep their identity.
• Add SSR handling for iterable children, including nested iterables.
• Add hydration coverage to ensure SSR-rendered iterable children hydrate without replacing existing DOM text nodes.
• Add universal renderer coverage.

How did you test this change?

• pnpm --filter @solidjs/web exec vitest run test/runtime/insert.spec.js --reporter=dot
• pnpm --filter @solidjs/web exec vitest run --config vite.config.server.mjs test/server/iterable-children.spec.tsx --reporter=dot
• pnpm --filter @solidjs/web exec vitest run --config vite.config.hydrate.mjs test/hydration/iterable-children.spec.tsx --reporter=verbose
• pnpm --filter @solidjs/universal exec vitest run test/runtime-insert.spec.js --reporter=dot
• pnpm --filter @solidjs/signals types
• pnpm --filter @solidjs/web types

[changeset-bot]

🦋 Changeset detected

Latest commit: c71ea56

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

This PR includes changesets to release 11 packages

Name	Type
@solidjs/signals	Patch
@solidjs/universal	Patch
@solidjs/web	Patch
test-integration	Patch
@solidjs/babel-plugin	Patch
@solidjs/element	Patch
@solidjs/h	Patch
@solidjs/html	Patch
@solidjs/compiler	Patch
@solidjs/diagnostics	Patch
solid-js	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codspeed-hq]

Merging this PR will improve performance by 18.14%

⚡ 1 improved benchmark
✅ 141 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	color-picker: 14 colors (renderToString)	771.4 µs	653 µs	+18.14%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing nickshiro:fix/3054-iterable-children (c71ea56) with next (8cfa272)}

[ryansolid]

Quick note on timing so this isn't read as being ignored: this and #3342 are both compiler/runtime-behavior changes carried over from the dom-expressions backlog, and they need a real design-level review rather than a quick pass — the babel plugin and the native compiler have to move together, and the size scenarios will shift. Ryan is prioritizing rc release-stability fixes right now, so these are deliberately queued behind that work. No action needed on your side in the meantime.

— Claude via Cursor