perf(wasi): optimize data plane and extend host ABI to v3 (#2455)

Overhaul the WASI guest data plane for throughput and add the host
capabilities it relies on. The externally driven Tokio runtime now
runs its timer pre-turn only when a tracked deadline has expired,
and all WASI-reachable timers (STUN, port mapping, WebClient, UDP
flow cleanup) go through the portable time facade so conditional
timer driving cannot starve them.

Data plane:

- Move read/write deadlines onto TCP and UDP resources with one ABI
  setter per direction, reuse a single expiration timer per
  resource, and drop timeout arguments from the four hot data-plane
  submissions (ABI v3). Checked absolute instants treat
  unrepresentable finite timeouts as unbounded instead of panicking.
- Batch host traffic: vectored TCP frame writes combine queued
  slices into one host operation, and reads request a bounded 64
  KiB while retaining excess bytes in the stream buffer.
- Complete TCP writes inside the guest with cancellation-safe
  writes, reporting the completed prefix before honoring
  cancellation or timeout so hosts never replay bytes.
- Repoll smoltcp egress immediately on zero poll delay, enlarge
  virtual UDP receive queues to 128 KiB payload with 128 metadata
  slots, and bound UDP session receive buffers to 8 KiB plus one
  byte while keeping oversized-datagram detection.

Host integration:

- Add optional algorithm-neutral AEAD seal/open imports with the
  ring backend as fallback, and pin the ring AES-128-GCM wire vector
  so the Go host stays interoperable.
- Forward instance events to hosts through one best-effort,
  synchronous, non-blocking import.
- Add a repository-owned build entry point for the Go host artifact:
  Binaryen 131 at -O4 with cached, SHA-256-verified official
  archives.

CONTRIBUTING.md

@@ -113,6 +113,17 @@ cargo build --release --target x86_64-pc-windows-msvc        # Windows x86_64
 
 Build artifacts: `target/[target-triple]/release/`
 
+### Building the WASI core
+
+```bash
+script/build-wasi-core.sh
+```
+
+This builds the `easytier-core` Go-host profile for `wasm32-wasip1`, then
+optimizes it with the pinned official Binaryen release. Binaryen is downloaded
+once into `target/binaryen/` and verified by SHA-256; set `WASM_OPT` to use an
+existing matching binary.
+
 ### Building GUI
 
 ```bash
@@ -222,4 +233,4 @@ Feel free to:
 - Join our community discussions
 - Reach out to maintainers
 
-Thank you for contributing to EasyTier! 
+Thank you for contributing to EasyTier!