Move the AWS RUM sink into the core's rum entry point #42

Part of the core extraction (#35). Move the AWS RUM sink into the core's /rum entry point.

Scope

• The sink moves to @absaoss-cps/ui-watchtower/rum (aws-rum-web stays an optional peer, loaded lazily); the Angular /rum entry extends it.
• A spec keeps the Angular package's private helper copies from drifting from the main entry's.
• Coverage gate: 90% on every metric, in both packages.

Done when

• The sink's behaviour is unchanged and npm run test:coverage passes with the gate.