Pass top-level region through ApplicationLoadBalancer child resources and lookups

[corymhall]

Parent: #1933

awsx:lb:ApplicationLoadBalancer exposes a top-level region input, but the implementation does not consistently thread that region through child resources and region-sensitive lookups/defaults.

Known child/defaulted paths to audit:

• subnet lookups
• default VPC lookup path
• default security group creation
• root aws.lb.LoadBalancer resource
• default target group
• listeners

Implementation references:

• awsx/schema-types.ts (ApplicationLoadBalancerArgs.region)
• awsx/lb/applicationLoadBalancer.ts
• awsx/ec2/getDefaultVpc.ts

Acceptance criteria:

• top-level region is passed to all ALB child AWS resources that support it
• region-aware lookups/defaults used by ALB receive the top-level region when present
• targeted tests cover the changed behavior