feat(clients): add Zig client SDK with C ABI

New Zig HTTP client for verisim-api with thin C ABI wrapper so other
languages (including hypatia's Rust data layer) can depend on a single
FFI surface instead of each rolling their own HTTP client.

- src/root.zig — native Zig client (create/get/update/delete octad, health, vql_execute)
- src/c_abi.zig — extern "C" wrappers: verisimdb_health, verisimdb_create_octad, …, verisimdb_free
- include/verisimdb_client.h — hand-authored C header for downstream consumers
- tests/test_client.zig — 9 integration tests against live verisim-api
- build.zig + build.zig.zon — produces libverisimdb_zig.{a,so}

Aligns with the hyperpolymath ABI/FFI standard (Idris2 ABI + Zig FFI).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

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