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

[abalmush]

Summary

#195

• Extract framework-agnostic data processing, types, guards, and message handling from @a2ui/lit into a new @a2ui/core package
• Enable React, Vue, and other framework developers to integrate A2UI without reimplementing core functionality
• Maintain backwards compatibility by re-exporting core types from @a2ui/lit and @a2ui/angular

Problem

External developers who want to integrate A2UI with their existing component libraries (React, Vue, or custom design systems) currently have to rebuild core functionality from scratch. The data processing, path resolution, and message handling logic lives inside @a2ui/lit, which means:

• React teams must reimplement A2uiMessageProcessor and all the JSON Pointer utilities
• Vue teams do the same
• Anyone not using Lit or Angular starts from zero

Solution

Create a new @a2ui/core package containing:

Export	Description
A2uiMessageProcessor	Processes server-to-client messages, manages surfaces and data model
Types	All A2UI component types (Text, Button, Card, etc.)
Primitives	StringValue, NumberValue, BooleanValue types
Guards	Runtime type checking functions (isResolvedText, isResolvedButton, etc.)
Events	Event type definitions

Changes

New package: renderers/core/

• src/processor/model-processor.ts - Message processor implementation
• src/types/ - All type definitions
• src/guards/ - Type guard functions
• src/events/ - Event definitions

Modified: renderers/lit/

• Now depends on @a2ui/core
• Re-exports core types for backwards compatibility
• Removed duplicated code (~1800 lines)

Modified: renderers/angular/

• Now depends on @a2ui/core
• Added @a2ui/core to allowedNonPeerDependencies

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[gemini-code-assist]

Code Review

This is a great refactoring that extracts the framework-agnostic logic into a new @a2ui/core package. This will significantly improve reusability for other frameworks. The new package is well-structured and comes with a comprehensive test suite.

My review focuses on the new core logic and I've identified a few areas for improvement:

• A potential bug in a regex used for path resolution.
• An inconsistency between a type definition and its corresponding type guard.
• Opportunities to refactor repetitive code and clarify the public API of the new package.

Overall, this is a solid piece of work. Addressing these points will improve the robustness and maintainability of the new core package.