Add copy(&mut self) methods backed by wc_Sha256Copy/wc_Sha384Copy
for transcript snapshotting (e.g. TLS handshake hashing).
Uses &mut self (not Clone trait) because wc_ShaCopy may mutate src
on some platforms. Initializes dst before calling Copy to avoid
passing uninitialized memory to wc_ShaFree inside the C function.
Includes unit tests verifying independent copy semantics with
externally computed SHA-256/SHA-384 test vectors.