Add emscripten_dns_lookup_async / emscripten_dns_lookup_result

[guybedford]

This adds an asynchronous getaddrinfo on top of the getaddrinfo body from #27693, so a real DNS lookup under -sNODERAWSOCKETS can be awaited without blocking from any stack. Recreates the async API from #27182 on the landed approach.

getaddrinfo() under NODERAWSOCKETS can only wait for a node:dns lookup from a sync-proxied pthread or with ASYNCIFY/JSPI; on the main thread of a plain build it returns EAI_AGAIN. Event-loop reactors need a non-blocking form.

• emscripten_dns_lookup_async(node, service, hints) takes the same inputs and returns an fd that becomes readable (poll/select/epoll) once the lookup completes; numeric addresses and validation errors are readable on return. A pending lookup holds the runtime like a timer.
• emscripten_dns_lookup_result(fd, &res) reads the outcome: 0 with a newly allocated addrinfo list in *res (freed with freeaddrinfo), an EAI_* code, or EAI_AGAIN while pending. Nothing is allocated until a result is read, so the fd owns no C memory: dup shares the lookup and close has nothing to free.
• The fd is a plain FS stream with its own poll and wait-queue node, so it needs nothing from SOCKFS. Without NODERAWSOCKETS the API works the same, resolving synchronously.

The getaddrinfo body becomes $getAddrInfo, which allocates nothing and returns an EAI_* code or a descriptor (with a lookup thunk for a hostname under NODERAWSOCKETS); $writeAddrInfo mints the addrinfo list at the point ownership passes to the caller. getaddrinfo keeps its proxied / Asyncify.handleAsync / EAI_AGAIN dispatch unchanged.

Tests: sockets_node.test_noderawsockets_dns_async (with PROXY_TO_PTHREAD: numeric and error results readable at once, repeated reads each minting their own list, close without reading, dup sharing the lookup, close while pending, and a real localhost lookup awaited via non-blocking poll retries from the main thread or a blocking poll from the pthread) and other.test_dns_lookup_async for the default build.

Made with AI assistance under my review