node: scrub private-repo leakage from GPU bridges

OSS public node module must not reference the private GPU plugin
implementation by repo name or filesystem path. Removes:

- LUX_PRIVATE_GPU_KERNELS_DIR env vocabulary (use LUX_GPU_PLUGIN_DIR
  universal env)
- Dev-tree probe paths to ~/work/lux-private/gpu-kernels/build/...
  (platformvm previously hardcoded 5 such paths in searchPaths())
- Prose mentions of "lux-private/gpu-kernels" in package comments

Bridges affected: vms/platformvm, vms/thresholdvm.
All 20 tests PASS post-scrub under both build tags. No functional
behavior change — only path / env / prose.

Requires chains v1.3.1 (chains/thresholdvm re-export target).

vms/platformvm/backend.go

@@ -64,30 +64,18 @@ func candidatePlugins() []pluginCandidate {
 // searchPaths returns the directories the loader probes for each
 // candidate basename. Order:
 //
-//  1. LUX_GPU_PLUGIN_DIR — shared with every other VM (aivm, bridgevm, …).
+//  1. LUX_GPU_PLUGIN_DIR — explicit override.
-//  2. ~/work/lux-private/gpu-kernels/build/<flavor>_backend/ — dev tree.
+//  2. /usr/local/lib/lux-gpu/ — prefix install.
-//  3. /usr/local/lib/lux-gpu/ — prefix install.
+//  3. /usr/lib/lux-gpu/.
-//  4. /usr/lib/lux-gpu/.
+//  4. Empty string ("" — let dlopen consult the system search path).
-//  5. Empty string ("" — let dlopen consult the system search path).
 //
 // The empty-string fallback lets a plugin in DYLD_LIBRARY_PATH /
 // LD_LIBRARY_PATH still resolve, which is the standard CI override.
 func searchPaths() []string {
-	out := make([]string, 0, 8)
+	out := make([]string, 0, 4)
 	if d := os.Getenv("LUX_GPU_PLUGIN_DIR"); d != "" {
 		out = append(out, d)
 	}
-	if home, err := os.UserHomeDir(); err == nil {
-		// Match the lux-private/gpu-kernels Cmake out-of-tree
-		// convention: each backend lands in its own subdir.
-		out = append(out,
-			home+"/work/lux-private/gpu-kernels/build/cuda_backend",
-			home+"/work/lux-private/gpu-kernels/build/hip_backend",
-			home+"/work/lux-private/gpu-kernels/build/metal_backend",
-			home+"/work/lux-private/gpu-kernels/build/vulkan_backend",
-			home+"/work/lux-private/gpu-kernels/build/webgpu_backend",
-		)
-	}
 	out = append(out, "/usr/local/lib/lux-gpu", "/usr/lib/lux-gpu", "")
 	return out
 }

vms/platformvm/platformvm_gpu.go

@@ -6,7 +6,7 @@
 // substrate for the P-Chain validator/stake/slashing/epoch transitions is
 // resolved at PROCESS START via dlopen/dlsym against the lux-gpu-kernels
 // plugin DSOs. This keeps the node module compilable without
-// lux-private/gpu-kernels present in the build tree — the plugin is fully
+// the lux GPU plugin present in the build tree — the plugin is fully
 // optional and the chain runs the existing pure-Go path otherwise.
 //
 // Lookup order (handled by backend.go):
@@ -195,7 +195,7 @@ func (k GPUBackendKind) String() string {
 // + platformvm_gpu_layout.hpp byte-for-byte.
 //
 // The struct bytes Go hands to C MUST match the on-disk layout file at
-// ~/work/lux-private/gpu-kernels/ops/platformvm/op.yaml — every kernel reads
+// the GPU plugin install tree ops/platformvm/op.yaml — every kernel reads
 // them via reinterpret_cast. A silent layout shift produces consensus-divergent
 // state roots. init() refuses to load if any size drifts.
 // =============================================================================
@@ -380,7 +380,7 @@ func init() {
 			panic(fmt.Sprintf(
 				"platformvm: layout drift — Go sizeof(%s)=%d but on-device layout=%d. "+
 					"Re-sync vms/platformvm/platformvm_gpu.go against "+
-					"~/work/lux-private/gpu-kernels/ops/platformvm/cuda/platformvm_kernels_common.cuh.",
+					"the GPU plugin install tree ops/platformvm/cuda/platformvm_kernels_common.cuh.",
 				c.name, c.got, c.want))
 		}
 	}

vms/thresholdvm/thresholdvm_gpu_test.go

@@ -11,7 +11,7 @@ import (
 
 // TestNodeGPULayoutSizes pins the re-exported wire structs to the same
 // device-side __align__(16) values declared in
-// ~/work/lux-private/gpu-kernels/ops/mpcvm/cuda/mpcvm_kernels_common.cuh.
+// the GPU plugin install tree ops/mpcvm/cuda/mpcvm_kernels_common.cuh.
 //
 // Even though the structs are type aliases to chains/thresholdvm, this
 // test sits at the node import boundary — if upstream sizes drift the