refactor: extract framework-agnostic core into @a2ui/core package

Extract the data processing, type definitions, guards, and message
handling logic from @a2ui/lit into a new @a2ui/core package. This
enables React, Vue, and other framework developers to integrate A2UI
without reimplementing core functionality.

The new @a2ui/core package includes:
- A2uiMessageProcessor for processing server-to-client messages
- Type definitions for all A2UI components and primitives
- Type guards for runtime type checking
- Event type definitions

The @a2ui/lit and @a2ui/angular packages now depend on @a2ui/core
and re-export its types for backwards compatibility.

Author: abalmus-celonis

renderers/angular/ng-package.json

@@ -4,5 +4,5 @@
   "lib": {
     "entryFile": "src/public-api.ts"
   },
-  "allowedNonPeerDependencies": ["markdown-it", "@a2ui/lit"]
+  "allowedNonPeerDependencies": ["markdown-it", "@a2ui/lit", "@a2ui/core"]
 }

renderers/angular/package-lock.json

@@ -5,6 +5,7 @@
     "build": "ng build && node postprocess-build.mjs"
   },
   "dependencies": {
+    "@a2ui/core": "file:../core",
     "@a2ui/lit": "file:../lit",
     "markdown-it": "^14.1.0",
     "tslib": "^2.3.0"