- Add bux_mutex_* and bux_rwlock_* C runtime functions
- Add library/std/Sync.bux with Mutex and RwLock wrappers
- Add examples/sync.bux — threaded counter with Mutex
- Update docs/Stdlib.md with Sync section and example
- Update README.md feature table
- C runtime (OpenSSL): bux_sha256, bux_hmac_sha256, bux_random_bytes,
bux_base64_encode, bux_base64_decode, bux_bytes_to_hex
- library/std/Crypto.bux: Crypto_Sha256, Crypto_HmacSha256,
Crypto_HmacSha256Raw, Crypto_RandomBytes, Crypto_Base64Encode,
Crypto_Base64Decode
- examples/jwt.bux: full JWT creation with HS256 signature
- Link with -lcrypto in C backend