fix(algebra): return the point itself when folding a single point

[0xShadowX]

MultiScalarMul with algopts.WithFoldingScalarMul() returns P0 + [gamma]P0 instead of P0 when it is called with a single point:

res, _ := cr.MultiScalarMul([]*AffinePoint[T]{p0}, []*emulated.Element[S]{gamma}, algopts.WithFoldingScalarMul())
// expected: p0 (gamma^0 * p0), got: p0 + gamma*p0

The folding branch computes sum gamma^i * P[i] Horner-style: it starts with [gamma]P[n-1], folds P[n-2]..P[1] in the loop, then adds P[0]. For n == 1 the loop body never runs, so P[0] is both the starting point and the final addend. For n >= 2 the result is correct.

The same code is in four places, all fixed the same way (early return of the single point):

• std/algebra/emulated/sw_emulated (Curve.MultiScalarMul)
• std/algebra/emulated/sw_bls12381 (G2.MultiScalarMul)
• std/algebra/native/sw_bls12377 (Curve.MultiScalarMul)
• std/algebra/native/sw_grumpkin (Curve.MultiScalarMul)

Tests:

go test -count=1 -run TestMultiScalarMulFoldedSinglePoint ./std/algebra/emulated/sw_emulated/ ./std/algebra/emulated/sw_bls12381/ ./std/algebra/native/sw_bls12377/ ./std/algebra/native/sw_grumpkin/
go test -short -run 'MultiScalar|Folded|MSM' ./std/algebra/...
go test -short ./internal/stats/ ./std/commitments/kzg/

TestMultiScalarMulFoldedSinglePoint is added in each package and fails on master in all four; existing MSM tests and stats are unchanged.

• regression test in each affected package
• no generated files touched

---

Note

Medium Risk
Correctness fix in folded multi-scalar multiplication used in ZK circuits; wrong results could affect proofs that call MSM with one point, but the change is a small edge-case guard with per-package regression tests.

Overview
Fixes MultiScalarMul with algopts.WithFoldingScalarMul() when there is only one point: the Horner-style folding path used to return P₀ + [γ]P₀ instead of γ⁰·P₀ = P₀, because the same point was both the initial [γ]P accumulator and the final addend.

The folding branch now early-returns p[0] when len(p) == 1 in all four curve wrappers: emulated sw_emulated and sw_bls12381 G2, and native sw_bls12377 and sw_grumpkin.

TestMultiScalarMulFoldedSinglePoint is added in each package to lock in the single-point behavior via test.IsSolved.

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