feat(demos): gate behind -tags demos (grpc opt-in only)

The 8 demo mains import github.com/hanzoai/base which pulls grpc
transitively via the legacy Google Cloud SDK chain. grpc is opt-in
only across the Lux stack, so demos follow the same discipline:
build with -tags demos.

Default fhe build: 0 grpc / 0 google cloud.

cmd/demos/auction/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // Copyright (C) 2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.
 

cmd/demos/compliance/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // Copyright (C) 2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.
 

cmd/demos/darkpool/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // Copyright (C) 2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.
 

cmd/demos/marketmaker/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // Copyright (C) 2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.
 

cmd/demos/nav/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // Copyright (C) 2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.
 

cmd/demos/proof/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // Copyright (C) 2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.
 

cmd/demos/regulated-vault/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // regulated-vault — Private app where operators can't see user data,
 // but regulators can force-decrypt via MPC threshold approval.
 //

cmd/demos/voting/main.go

@@ -1,3 +1,5 @@
+//go:build demos
+
 // Copyright (C) 2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.