Add mdata-client: Rust rewrite of SmartOS metadata client

[nwilkens]

Summary

Rust implementation of the SmartOS metadata client (mdata-get, mdata-put, mdata-list, mdata-delete), replacing the existing C implementation in triton/mdata-client.

• Implements both V1 and V2 metadata protocols with automatic negotiation
• Supports Unix domain sockets (SmartOS zones) and serial ports (KVM/HVM guests)
• Platform detection tries zone socket paths first, then serial ports — works on illumos, Linux, and BSD
• ~1,100 lines of Rust; uses base64, crc32fast, and libc crates
• Uses poll() for I/O timeouts across all platforms (vs platform-specific event ports/epoll/kqueue in the C version)
• Integrates into the monitor-reef workspace

Test plan

Tested on two live instances with full CRUD operations:

SmartOS zone (smartos-build1, base-64-lts@24.4.1 — Unix socket transport):

# mdata-list → empty (exit 0) ✓
# mdata-put test_key "hello from rust" → exit 0 ✓
# mdata-get test_key → "hello from rust" (exit 0) ✓
# mdata-list → "test_key" (exit 0) ✓
# mdata-delete test_key → exit 0 ✓
# mdata-get test_key → "No metadata for 'test_key'" (exit 1) ✓
# mdata-get sdc:nics → full NIC JSON (exit 0) ✓

Ubuntu 24.04 bhyve VM (edge_foundry — serial port transport):

# mdata-list → "root_authorized_keys\nroot_pw" (exit 0) ✓
# mdata-get sdc:nics → full NIC JSON (exit 0) ✓
# mdata-put test_key "hello from ubuntu rust" → exit 0 ✓
# mdata-get test_key → "hello from ubuntu rust" (exit 0) ✓
# mdata-delete test_key → exit 0 ✓
# mdata-get test_key → "No metadata for 'test_key'" (exit 1) ✓

Cross-tool interoperability (SmartOS): Native C mdata-put → Rust mdata-get and vice versa — both directions work correctly.

🤖 Generated with Claude Code