Prevents cmake from discovering the parent src/CMakeLists.txt which
has a broken project() call that causes install-export errors.
Also update test count to ~600.
Test count updated to ~4277 CTest runs across 8 algorithm variants.
Document post-quantum KEX design: K_ENCODING_STRING flag, vendor
crypto (sntrup761/SUPERCOP, libcrux/mlkem768), conditional
ssh-internal.h includes for test injection.