Scaffold the ui-watchtower core package #38

Part of the core extraction (#35). Scaffold the @absaoss-cps/ui-watchtower package next to the Angular one, without moving the engine yet.

Scope

• Package, build (ng-packagr), TypeScript and Jest setup, API snapshot and public-api spec for projects/ui-watchtower.
• The Angular package lists the core as a dependency and resolves it from the workspace while developing.
• CI runs the core's tests and reports its coverage separately.

Done when

• Both packages build and their tests pass; the Angular package's behaviour and public API are unchanged.