Add TypeScript types for core

**Problem**
When working with store, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

The issue names the core and store areas but no files, entry points, or tests. First clarify whether the requested work is TypeScript type coverage, improved store error messages, or both, then locate the relevant core/store code and existing error tests. Done should mean the agreed types and descriptive error behavior are implemented and covered by tests.

Written by the indexing model from the issue text.