P3.5: acc_437c0 = data shuffle (re/im repack + zero-fill), not arithmetic; full butterfly = twiddle load + shuffle + mult + combine

notes_giant_fft.md

@@ -115,6 +115,13 @@ for dB, log/gamma in LUT curve), so it feeds bit-exact level→dB.
 - actual twiddle usage for small N via FUN_180039b00 sin-table
 So P3 scope is unchanged (butterfly + plan gen), NOT the ln kernels.
 
+### acc_437c0 (called from stage_bfc0) = DATA SHUFFLE, not arithmetic
+0x1800437c0 does interleaved<->split re/im repacking: movsd (write one double) + unpckhpd
+(unpack high) + writing 0.0 (xorps xmm0) — i.e. real->complex zero-fill / deinterleave.
+So the butterfly stage = twiddle load (0x140c40) + shuffle (0x437c0) + elementwise mult
+(0x8440) + combine. Transcribing the full stage requires the shuffle layout too.
+=> Full bit-exact FFT body confirmed large: shuffle + butterfly + plan gen (multi-week).
+
 ## STATUS / scope
 Full bit-exact FFT = plan generator (0x2f980 split-radix index algebra) + stage kernels
 (0xbfc0/0xc5e0 butterfly) + twiddle. ln (vlog) now done. FFT butterfly + plan gen remain.