Add public, tier, node fields to ServiceDef

RouteDef gains Public field (bool) for edge routing. ServiceDef gains
Tier field. Node validation relaxed: defaults to tier=worker when both
node and tier are empty (v2 compatibility).

ToProto/FromProto updated to round-trip all new fields. Without this,
public=true in TOML was silently dropped and edge routing never triggered.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/servicedef/servicedef_test.go

@@ -119,14 +119,8 @@ func TestValidation(t *testing.T) {
 			},
 			wantErr: "service name is required",
 		},
-		{
-			name: "missing node",
-			def: &ServiceDef{
-				Name:       "svc",
-				Components: []ComponentDef{{Name: "api", Image: "img:v1"}},
-			},
-			wantErr: "service node is required",
-		},
+		// v2: missing node no longer errors — defaults to tier=worker.
+		// Tested separately in TestValidationNodeTierDefault.
 		{
 			name: "empty components",
 			def: &ServiceDef{