product: implement the jk_* FFI API in ffi/zig/src/main.zig to match the integration tests

[hyperpolymath]

The integration tests target the intended API (ContentHash, KeyId, OblitProof, Error, jk_init, jk_close, ...) per include/januskey.h + Layout.idr, while main.zig is generic template scaffold (januskey_init/januskey_process(u32), "Add your fields here"). They have never matched — the "Zig FFI Build + Test" gate's test step is red until this lands (build itself fixed in PR #55: libc link, struct Handle, test module import).

https://claude.ai/code/session_01GJatEm2TVFSTBEkKXmserJ