Replace ntt_core's 256x12 LUT register file (multi-port async read, ~18.5k
LUT) with coeff_bram_4way: 4 sd_bram banks, conflict-free banking function
bank(a)={a[0], ^a[7:1]}, local(a)=a[7:1]. The dual-butterfly's four accesses
{j, j+1, j+len, j+1+len} map to four distinct banks for every layer_len in
{2..128}, so each bank needs only 1 read + 1 write/cycle (single buffer, no
ping-pong). Trades ~13k LUT for +4 BRAM (device BRAM at ~6%).
Pipeline gains one stage for the synchronous BRAM read; writeback address
delay deepened to 9. coeff_out is driven combinationally from the BRAM read
during OUTPUT; unused read ports mirror port 0 outside compute so the read
crossbar arbitration is not polluted by stale addresses.
Verified byte-exact vs NIST KAT: NTT standalone TB 19/19 (fwd+inv), Encaps
9/9, Decaps 9/9 (incl. reject + DECOMP), KeyGen, hello_world e2e — all PASS.
Cycle counts unchanged (ENC_N +4 for BRAM latency).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
