docs(examples): add Zig guest

[lntutor]

Summary

• add a minimal Zig 0.16.0 guest that imports only nx.host_log_v2 and exports memory plus run
• add a Node ABI smoke test that inspects and executes the compiled WebAssembly module
• document build, test, runtime usage, raw ABI details, and current limitations
• build and test the Zig example in the existing WASM CI job with pinned tooling

Verification

• examples/guest_zig/build.sh
• node examples/guest_zig/test.mjs
• wasm-tools validate examples/guest_zig/guest.wasm
• ./target/debug/nx run examples/guest_zig/guest.wasm
• cargo test --workspace -q (498 passed)
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• git diff --check

Closes #59

[lntutor]

Current head 1de9bb3552633113507951e2c97dd6cea4374a0c is mergeable and the visible CI surface on this exact SHA is fully green: cross-platform Check, Clippy, and Test jobs on Ubuntu, macOS, and Windows; Format; Build WASM Examples; CLI Multi-process Smoke; Benchmark Comparator Tests; Ubuntu Profiling Artifacts; Benchmark Regression Gate; and the aggregate CI Success check all completed successfully. There are no review threads or review-body comments open on the current PR state, so if this Zig guest example still matches the intended direction, it looks ready for maintainer review/merge when convenient.