add jetstream smoke test and fix publish-docs for zig 0.16
- add scripts/jetstream_smoke.zig with `zig build smoke` step
- fix publish-docs.zig: posix.getenv → std.c.getenv, std.time.timestamp
→ Io.Timestamp, std.fs.cwd → Io.Dir, XrpcClient.init signature
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>