fix(compiler,ui): wrap multi-child component children in DocumentFragment [#2821]

[viniciusdacal]

Summary

Closes #2821.

RouterContext.Provider (and any other component that treats children as a renderable slot — Suspense, ErrorBoundary, ThemeProvider, …) crashed at mount when given multiple JSX children:

<RouterContext.Provider value={router}>
  <aside>sidebar</aside>
  <main>…</main>
</RouterContext.Provider>

Uncaught TypeError: Failed to execute 'appendChild' on 'Node': parameter 1 is not of type 'Node'.
    at __append (chunk-…:288:10)
    at App (src/app.tsx:…)

Root cause: the compiler emitted Component({ children: () => [a, b] }) for multi-child components. Consumers called children() expecting a single Node, got an array, and downstream appendChild rejected it. The dev-only guard in Context.Provider that was supposed to produce a friendly error was gated on process.env.NODE_ENV, which is not polyfilled in the browser — so users saw the generic DOM error instead.

Public API Changes

Fixed

• Compiler — multi-child component children now compile to a DocumentFragment-returning thunk (same shape fragments use), so <Provider><a/><b/></Provider> behaves like <Provider><><a/><b/></></Provider>. No more "children must have a single root element" requirement.
• Context.Provider — array results from hand-written callers are flattened via resolveChildren into a DocumentFragment, instead of throwing. Replaces the unreliable dev-only throw.

Breaking

None. The previous behavior was a crash; the new behavior is correct rendering.

Test plan

• New compiler tests: component_children_multiple now asserts createDocumentFragment output; new component_member_expression_multi_child_uses_fragment reproduces the exact RouterContext.Provider case from router: RouterContext.Provider with multiple JSX children throws generic appendChild error #2821.
• Provider unit tests: array-children → fragment; primitives coerce to text; null/undefined/boolean filtered; direct (non-thunk) arrays; nested arrays and thunks flatten.
• cargo test --all — 1127 compiler tests + all sub-crates pass.
• cargo clippy --all-targets -- -D warnings — clean.
• cargo fmt --all -- --check — clean.
• packages/ui/ typecheck clean.
• context.test.ts — 40/40 tests pass.
• Full GitHub CI (monitored after push).

Files

• native/vertz-compiler-core/src/jsx_transformer.rs — build_children_thunk now emits a fragment for ≥2 children.
• packages/ui/src/component/context.ts — Provider flattens array results via resolveChildren.
• packages/ui/src/component/tests/context.test.ts — updated + new tests.
• .changeset/fix-multi-child-component-fragment.md — patch changeset for @vertz/ui and @vertz/native-compiler.

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