feat: local-socket-halves — the local feature (TCP/UDP/unix halves functions)

- src/local/mod.rs (feature = "local" -> tokio/net): dial_tcp (5s
  timeout), connect_udp (ephemeral bind + connect + the FRAMED UdpHalf
  adapter), dial_unix (OQ-TN-14 in v1), bind_tcp +
  TcpListenerHalves::accept_loop (the listen shape's assembly half),
  local_dial() (one DialFn covering all three substrates).
- UdpHalf truncation fails loud (OQ-TN-13): poll_read recvs into a
  scratch buffer first and size-checks against the caller's buffer —
  an over-size datagram is io::ErrorKind::InvalidData, never a silent
  truncation (tokio's poll_recv-into-ReadBuf would truncate).
- No socket type appears outside src/local/ (convention 16); the
  default crate stays wasm-clean; stdio bridging is NOT here (alktty
  owns process stdio).
- Tests (tests/local_halves.rs, 7, feature-gated): TCP/UDP/unix dial
  round-trips through the real establisher path, the 1400-byte MTU
  datagram, the truncation fail-loud probe, dial-refusal ->
  dial_failed, listen producer over a real TCP listener end-to-end.

Verified: cargo test green (default + --all-features, 44 + 7),
clippy -D warnings (default + all-features + wasm32), fmt clean,
cargo check --target wasm32-unknown-unknown passes.

Cargo.toml

@@ -16,6 +16,7 @@ name = "alktunnels"
 
 [features]
 default = []
+local = ["tokio/net"]
 
 [dependencies]
 alkcall = "0.7.0"