Add build option -Denable-liboqs to conditionally enable post-quantum crypto.
When disabled, uses liboqs_stub.zig which provides stub implementations
that return ML_KEM_NotAvailable error.
Changes:
- build.zig: Add enable_liboqs option and liboqs module selection
- pqxdh.zig: Use liboqs.isAvailable() runtime check for PQ operations
- soulkey.zig: Use pqxdh.enable_pq to conditionally generate ML-KEM keys
- liboqs_stub.zig: Stub implementations when liboqs not linked
- liboqs_real.zig: Real C FFI bindings when liboqs is linked
Fixes P0 security audit issue: liboqs Build Breakage
Markus Maiwald
