pub fn lookup(win: &[AffinePoint], k: i32) -> AffinePointExpand description
Window lookup: returns the affine point matching the signed digit k.
k > 0 selects k * P, k < 0 selects -k * P, and k == 0 returns
the affine neutral (0, 0). Mirrors the upstream Go Lookup byte-for-byte.
Despite the constant-shape walk over win, both the per-entry copy and
the final negation branch on the digit’s value, so secret digits leak
through timing. A true constant-time variant requires masked-select Fp5
primitives and lands in Phase D alongside the secret-scalar Schnorr path.