wasip2-examples: Tokio example apps for wasm32-wasip2

[Copilot]

PR tokio-rs#7933 landed networking support for wasm32-wasip2. This adds a wasip2-examples/ crate that documents the current support scope and validates it with runnable examples.

What works on wasm32-wasip2

Feature	Status
current_thread runtime, tokio::spawn, tokio::select!	✅
tokio::time (sleep, interval, timeout)	✅
tokio::sync (mpsc, oneshot, Mutex, Semaphore, Barrier)	✅
TcpListener / TcpStream / TcpSocket	✅ requires tokio_unstable + wasmtime -Sinherit-network
UdpSocket (send/recv)	✅ requires tokio_unstable + wasmtime -Sinherit-network
rt-multi-thread, tokio::fs, tokio::process, DNS	❌
SO_LINGER, POLLHUP, broadcast, multicast, peek	❌

New crate: wasip2-examples/

Standalone crate (outside workspace) — only needs rt, macros, net, io-util, time, sync:

• 01-hello.rs — timers, mpsc/oneshot channels, spawn, select!, timeout; no network
• 02-tcp-echo.rs — TcpListener binds ephemeral port, echo-server task accepts, TcpStream client round-trips three messages
• 03-udp-echo.rs — two UdpSockets exchange datagrams with send_to/recv_from
• 04-concurrent-tasks.rs — work queue (mpsc), shared counter (Mutex), rate limiter (Semaphore), rendezvous (Barrier)

All four use #[tokio::main(flavor = "current_thread")] — the multi-thread runtime is unavailable on wasip2.

Running

rustup target add wasm32-wasip2

# No-network examples
RUSTFLAGS="--cfg tokio_unstable" cargo run --target wasm32-wasip2 --bin 01-hello

# Network examples (wasmtime must be in PATH)
RUSTFLAGS="--cfg tokio_unstable" \
  CARGO_TARGET_WASM32_WASIP2_RUNNER="wasmtime run -Sinherit-network" \
  cargo run --target wasm32-wasip2 --bin 02-tcp-echo

CI

Build + run steps added to the existing wasm32-wasip2 job in ci.yml.