Cgo wrapper hardening: bounds, validation, missing stubs

[JeremiahM37]

Fixes F-3021, F-1866, F-2546, F-1872, F-1875, F-2550, F-2553, F-2551, F-3023

Summary:

• ChaCha20Poly1305 appended-tag encrypt: size the working buffer to exactly len(inPlain)+tag so oversized caller buffers don't leave garbage between cipher and tag.
• Add NO_HMAC and NO_PWDBASED cgo stubs so builds without HMAC/HKDF/PBKDF2 compile.
• Reject sizes >math.MaxInt32 before the C.int cast in WolfSSL_read/write, curve25519 make_pub/make_priv, BIO_new_mem_buf, and X509 buffer wrappers (BIO can wrap to -1, OpenSSL's strlen sentinel).
• Reject iter <= 0 in Wc_PBKDF2 and require exactly CURVE25519_KEYSIZE on curve25519 export/shared_secret outputs.