INT-03 follow-up: socket recv/send/accept primitives — byte-level buffer marshalling #487

ADR-015 S6b shipped net_shutdown (lowers to wasi_snapshot_preview1.sock_shutdown; command-adapter bridges to wasi:sockets/tcp) — the on-ramp. Larger socket primitives sock_recv / sock_send / sock_accept need byte-level buffer and network-address marshalling that goes beyond what S6b shipped.

Pattern is settled: the byte-level wasm IR (I32Load8U/I32Store8 and siblings) landed in #339 alongside env_at/arg_at and is what unblocks this. Helper shape will mirror gen_str_at_via_get (lib/wasi_runtime.ml): allocate iovec + byte buffer, call the WASI import, copy bytes into a length-prefixed AS string for the recv side; for send, walk the AS string into an iovec; for accept, read the new fd + remote-addr fields.

Effect surface stays Net (reserved at S6b). Component adapter already bridges to wasi:sockets/tcp internally — no host-side wasi:sockets/* requirement.

Ledger: INT-03 follow-up (S2 important — not substrate-blocking; the on-ramp suffices for net_shutdown consumers today). Refs #180.