Add ST_N stage: forward NTT (ntt_core mode=0, no scaling) of s[0],s[1], e[0],e[1] in place (slots S0,S1,E0,E1). Per slot: stream 256 coeffs into ntt_core during LOAD, collect 256 outputs back to same slot. n_pending waits for core IDLE between slots. Verified vs ml-kem-r golden: 1024/1024 shat/ehat coeffs exact (17318 cyc).