backend: drop LUX_ prefix from env var — GPU_DISABLE

Per "no LUX_ in front of shit for no reason" — the kill switch is a
generic GPU-disable knob, not Lux-specific.

  LUX_GPU_DISABLE → GPU_DISABLE

All docs + comments updated. Behavior unchanged.

backend/backend.go

@@ -102,7 +102,7 @@ func CGoAvailable() bool { return cgoLinked }
 // GPUAvailable reports whether the luxfi/accel GPU substrate is reachable
 // in this process. First call lazily initialises accel via gpuhost; the
 // answer is cached afterwards. Returns false unconditionally when the
-// LUX_GPU_DISABLE kill switch is set (see disable.go).
+// GPU_DISABLE kill switch is set (see disable.go).
 func GPUAvailable() bool {
 	if gpuDisabled {
 		return false

backend/disable.go

@@ -8,23 +8,23 @@ import (
 	"strings"
 )
 
-// LUX_GPU_DISABLE is the process-wide operator kill switch for GPU
-// dispatch. Read exactly once at init; the value is then constant for
-// the lifetime of the process. Set to a truthy value
-// (1 / true / yes / on) to force every algorithm dispatcher to its CPU
-// path regardless of whether a device is present.
+// GPU_DISABLE is the process-wide operator kill switch for GPU dispatch.
+// Read exactly once at init; the value is then constant for the lifetime
+// of the process. Set to a truthy value (1 / true / yes / on) to force
+// every algorithm dispatcher to its CPU path regardless of whether a
+// device is present.
 //
 // This is orthogonal to GPUAvailable() — that probe answers "is there a
 // device", this knob answers "should we use it even if there is". Use
 // cases: GPU driver regression in prod, A/B rollout where some racks
 // stay on CPU, validators sharing a host with another tenant.
-const envGPUDisable = "LUX_GPU_DISABLE"
+const envGPUDisable = "GPU_DISABLE"
 
 var gpuDisabled = parseTruthy(os.Getenv(envGPUDisable))
 
-// GPUDisabled reports whether the LUX_GPU_DISABLE kill switch is set.
-// When true, GPUAvailable() returns false and IsGPU() therefore returns
-// false, forcing every dispatcher onto its CPU path.
+// GPUDisabled reports whether the GPU_DISABLE kill switch is set. When
+// true, GPUAvailable() returns false and IsGPU() therefore returns false,
+// forcing every dispatcher onto its CPU path.
 func GPUDisabled() bool { return gpuDisabled }
 
 // parseTruthy mirrors the convention used by strconv.ParseBool but

backend/fallback.go

@@ -16,7 +16,7 @@ import (
 type FallbackReason uint32
 
 const (
-	// FallbackDisabled — LUX_GPU_DISABLE is set; dispatcher routed to CPU.
+	// FallbackDisabled — GPU_DISABLE is set; dispatcher routed to CPU.
 	FallbackDisabled FallbackReason = iota
 	// FallbackUnsupported — host driver reported no usable device
 	// (cudaGetDeviceCount == 0, Metal device init failed, etc.).

backend/probe.go

@@ -26,7 +26,7 @@ type Snapshot struct {
 	// GPU reports whether luxfi/accel found at least one device. Always
 	// false when Disabled is true, regardless of the underlying probe.
 	GPU bool
-	// Disabled reflects the LUX_GPU_DISABLE operator kill switch.
+	// Disabled reflects the GPU_DISABLE operator kill switch.
 	Disabled bool
 	// GPUBackend names the active accel backend ("metal" / "cuda" /
 	// "webgpu") or "" when no GPU is available.