Add Direct Sockets API support for Isolated Web Apps (-sDIRECT_SOCKETS)

[maceip]

replaces the websocket-to-posix-socket proxy with chromes Direct Sockets API (TCPSocket TCPServerSocket UDPSocket) for real tcp/udp networking from wasm in isolated web apps

context on changes from #26344

rewrite that incorporates all review feedback from #26344 and expands scope to fully support the direct socket api - driven by A) young jedis annoying me about web transport and B) the archive org folks asking about tor-in-wasm this past weekend in berlin - with my janky syscall wiring plus this patch you get unbelievable perf across udp [incl session tickets] and tcp - shout outs to emscripten core devs and blink/v8 devs this shouldnt be possible

feedback addressed from #26344

feedback	fix
share fd allocator with FS / fd collision at 100 (@sbc100)	socket fds now allocated via FS.createStream() using the SOCKFS pattern
use autoAddDeps instead of repeating __deps (@sbc100)	added autoAddDeps(DirectSocketsLibrary '$DIRECT_SOCKETS')
use cDefs constants instead of hardcoded numbers (@sbc100)	all sock opt constants now in struct_info json and referenced via cDefs
eof vs error distinction in read path (copilot)	stream_ops read and readFromSocket check sock error before returning eof
parseSockaddr drops specific errno (copilot)	returns {errno: X} all callers propagate the specific error
feature detection for TCPSocket/UDPSocket (copilot)	abort() under ASSERTIONS -ENOSYS at runtime
bind-then-connect ignores local endpoint (copilot)	connect() passes sock localAddress/localPort to constructor opts
udp bind+connect leaks socket (copilot)	connect() closes existing sock udpSocket before creating new one
fd_close async issue (copilot)	fire-and-forget via stream_ops close fd freed synchronously
add link to direct sockets spec (@sbc100)	in file header

key architectural change: SOCKFS pattern

the original pr used a private fd allocator (nextFd: 100) - this version registers socket fds in emscriptens FS using FS.createNode() + FS.createStream() with custom stream_ops the same pattern SOCKFS uses for websocket-backed sockets - this means write(fd) and read(fd) route through direct sockets which is reqd by openssl (its socket BIO uses write()/read() not send()/recv())

stream_ops must be synchronous bc theyre called from js to js (FS.write -> stream_ops.write) not wasm to js so JSPI cant suspend:

• write: fire-and-forget via writer.write() returns byte count immediately
• read: consumes from recvQueue (filled by bg reader) throws EAGAIN if empty
• poll: checks recvQueue.length for readability

new syscalls (beyond #26344)

syscall	notes
setsockopt / getsockopt	TCP_NODELAY SO_KEEPALIVE SO_RCVBUF SO_SNDBUF IP_ADD_MEMBERSHIP IP_DROP_MEMBERSHIP IP_MULTICAST_TTL IPV6_JOIN_GROUP IPV6_LEAVE_GROUP
poll	via recvQueue length checks + async wait w timeout
pipe2 / socketpair	in-memory pipe buffers w FS-backed fds
fcntl64	F_GETFL / F_SETFL for O_NONBLOCK
ioctl	FIONBIO FIONREAD
write / read	via FS stream_ops (the SOCKFS pattern)
_emscripten_lookup_name	uses emscriptens std DNS (inetPton4 packed uint32)

files changed

file	change
src/lib/libdirectsockets.js	new all syscall impls
src/settings.js	add DIRECT_SOCKETS flag
src/modules.mjs	register libdirectsockets js when flag enabled
src/lib/libsyscall.js	guard default socket impls when DIRECT_SOCKETS active
src/lib/libwasi.js	fd_close path for direct socket fds
src/struct_info.json	added sock opt constants (SO_REUSEADDR TCP_NODELAY IP_ADD_MEMBERSHIP etc)
site/source/docs/porting/networking.rst	added direct sockets docs section

usage

emcc -sDIRECT_SOCKETS -sASYNCIFY -sPROXY_TO_PTHREAD -pthread app.c -o app.js

notes

• setsockopt stub in emscripten_syscall_stubs.c is declared weak so the js lib impl takes priority automatically no need to modify stubs file
• requires ASYNCIFY (or JSPI) - compile err if neither enabled
• doh dns resolution split out to follow-up pr per @sbc100 feedback
• pselect6 not impl yet - poll() covers most use cases and select() can be routed thru it

testing

• tcp: unmodified upstream tor full node (pure c) which pulls in openssl => compiled to wasm bootstraps 100% in ~15s in a chrome iwa:
  https://github.com/maceip/tor.iwa
• udp/quic/h3: ngtcp2 + wolfssl + nghttp3 stack achieves ~90% of native throughput:
  https://github.com/maceip/socket.iwa

web demo

demo.mp4