Wire up official bluesky-social/atproto-interop-tests as a lazy build dependency. Tests are embedded at comptime and only fetched when running zig build test. Tier 1 — syntax validation for Tid, Did, Handle, Nsid, Rkey, AtUri against valid/invalid fixture files. Tier 2 — crypto signature verification against 6 test vectors covering ES256/ES256K with valid low-S, invalid high-S, and invalid DER-encoded signatures. Tier 3 — MST key height computation (SHA-256 leading zero bits / 2) against 9 test vectors. Bumps version to 0.1.8. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>