dkg2: rename gpu fan-out files to dkg2_parallel.go (honest labelling)

The two build-tagged files (dkg2_gpu_default.go, dkg2_gpu_accel.go)
implemented a goroutine fan-out, NOT a GPU dispatch. Real GPU NTT for
the underlying ring math lives in luxfi/lattice/v7/gpu and is reached
via the consensus engine accel pipeline.

Collapse to a single file dkg2_parallel.go with no build tag. Default
OFF (matching the prior non-gpu build); tests opt in via SetDKG2GPUForTest.

The exported helper name SetDKG2GPUForTest is left as-is to avoid breaking
existing test callers; the legacy `GPU` in the name is documented as a
historical misnomer.

dkg2/dkg2_gpu_accel.go

@@ -1,13 +0,0 @@
-// Copyright (C) 2025-2026, Lux Industries Inc. All rights reserved.
-// See the file LICENSE for licensing terms.
-
-//go:build gpu
-
-package dkg2
-
-// On by default in the gpu build. The fan-out is pure Go (no CGO); the
-// luxfi/accel kernel slots in at the engine layer for the v0.6+
-// submission pipeline.
-func init() {
-	dkg2GPUEnabled = true
-}

dkg2/dkg2_gpu_default.go

@@ -1,11 +0,0 @@
-// Copyright (C) 2025-2026, Lux Industries Inc. All rights reserved.
-// See the file LICENSE for licensing terms.
-
-//go:build !gpu
-
-package dkg2
-
-// Off by default. Tests opt in via SetDKG2GPUForTest.
-func init() {
-	dkg2GPUEnabled = false
-}

dkg2/dkg2_parallel.go

@@ -0,0 +1,11 @@
+// Copyright (C) 2025-2026, Lux Industries Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package dkg2
+
+// dkg2GPUEnabled is the runtime toggle for the goroutine fan-out path of
+// the Pedersen DKG hot loop. Default off; tests opt in via
+// SetDKG2GPUForTest. This is not a GPU dispatch — real GPU NTT for the
+// underlying ring math lives in luxfi/lattice/v7/gpu and is reached via
+// the consensus engine accel pipeline.
+func init() { dkg2GPUEnabled = false }