std.crypto: add hybrid post-quantum/traditional key encapsulation

Hybrid KEMs combine a post-quantum secure KEM with a traditional
elliptic curve Diffie-Hellman key exchange.

The hybrid construction provides security against both classical and quantum
adversaries: even if one component is broken, the combined scheme remains
secure as long as the other component holds.

The implementation follows the IETF CFRG draft specification for concrete
hybrid KEMs:

https://datatracker.ietf.org/doc/draft-irtf-cfrg-concrete-hybrid-kems/

lib/std/crypto.zig

@@ -116,6 +116,7 @@ pub const dh = struct {
 
 /// Key Encapsulation Mechanisms.
 pub const kem = struct {
+    pub const hybrid = @import("crypto/hybrid_kem.zig");
     pub const kyber_d00 = @import("crypto/ml_kem.zig").d00;
     pub const ml_kem = @import("crypto/ml_kem.zig").nist;
 };