Per Day-2 progress doc the kernel bottleneck was scalar_mul (256 doubles +
~128 adds per scalar), not _ModInv. v3 swaps the binary double-and-add for
a windowed-base ladder: precompute table[0..15] = i*P affine on-device,
walk scalar 4 bits at a time MSB->LSB, cutting per-scalar adds from ~128
to ~63. Table loaded into __shared__ (1024 B) per block.
Benched on 3090-ai (best-of-3, --no-batch-inv):
n=10k: v1 4.22 Mkeys/s v3 2.18 Mkeys/s 0.52x (init overhead dominates)
n=100k: v1 7.48 Mkeys/s v3 9.29 Mkeys/s 1.24x
n=1M: v1 7.87 Mkeys/s v3 13.60 Mkeys/s 1.73x
Byte-identical against coincurve at n in {32, 1000, 10000, 100000, 1000000}.
CLI: --window-w 4 selects v3 (Day-3 canonical). w=8 reserved but stub-
rejected since device-side table init is register-stack-bounded at W <= 16.
Default no-flag behaviour stays v1 (Day-1) so the gpu-worker.lsp daemon
inherits the safe baseline until fox routes traffic to v3.
Companion progress doc at plans/form-A-day3-progress.md.
russell@unturf.com
ecfe27a46f