fix: separate run builders from generated workers

platform/service/distributions.go

@@ -44,6 +44,9 @@ func (svc *CoreService) GenerateRunDistributionForSession(sessionID string, requ
 	if err := svc.requireCompleteRuntimeBindings(user.ID, instance.ID, "run.generate.denied"); err != nil {
 		return domain.RunDistribution{}, err
 	}
+	if err := svc.promoteLegacyRunBinding(&instance); err != nil {
+		return domain.RunDistribution{}, err
+	}
 	builderEndpointID := instance.RunEndpointID
 	if strings.TrimSpace(instance.DeploymentTargetID) != "" {
 		builderEndpointID = instance.DeploymentTargetID
@@ -125,6 +128,27 @@ func (svc *CoreService) GenerateRunDistributionForSession(sessionID string, requ
 	return domain.CopyRunDistribution(distribution), nil
 }
 
+// promoteLegacyRunBinding reserves a server-scoped endpoint for a generated
+// Run before a legacy server first requests a distribution. Its existing
+// endpoint remains the trusted build target; reusing it in the package would
+// allow the generated Run to replace the builder registration.
+func (svc *CoreService) promoteLegacyRunBinding(instance *domain.ServerInstance) error {
+	if instance == nil || strings.TrimSpace(instance.DeploymentTargetID) != "" || (instance.State != domain.ServerInstanceStateDraft && instance.State != domain.ServerInstanceStateFailed) {
+		return nil
+	}
+	builderEndpointID := strings.TrimSpace(instance.RunEndpointID)
+	if builderEndpointID == "" {
+		return validationError("legacy Run generation requires a build target endpoint")
+	}
+	instance.DeploymentTargetID = builderEndpointID
+	instance.RunEndpointID = dedicatedRunEndpointID(instance.ID)
+	instance.UpdatedAt = svc.now()
+	if err := validator.ValidateServerInstance(*instance); err != nil {
+		return err
+	}
+	return svc.store.ServerInstances().Update(*instance)
+}
+
 func (svc *CoreService) GenerateClientManagerDistributionForSession(sessionID string, request domain.ClientManagerBuildRequest) (domain.ClientManagerDistribution, error) {
 	request = domain.CopyClientManagerBuildRequest(request)
 	if strings.TrimSpace(request.IdempotencyKey) == "" {