When a component inside a <Suspense> boundary suspends during SSR,
emit the fallback immediately and append the resolved content at the
end of the stream with an inline script that swaps it into place.
This avoids blocking the entire HTTP response on slow async operations.

- Add SsrContext to track deferred suspense boundaries
- Poll children once in VSuspense::render_into_stream; if Pending,
  stream fallback with boundary markers and spawn deferred rendering
- Flush deferred content with $YC swap script after main tree completes
- Thread SsrContext through all render_into_stream methods
- Add suspense_ssr example demonstrating the feature

In browser WASM, the scheduler defers via spawn_local, so poll!()
always returns Pending regardless of whether children truly suspend.
This caused hydration integration tests to fail because non-suspending
components were incorrectly rendered with fallback+deferred format.

Out-of-order streaming now only activates on native/WASI targets
where the scheduler is synchronous. On WASM-browser, the old inline
rendering behavior is preserved.