Add TypeScript types for core

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

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

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

Start by locating the core TypeScript type definitions and the existing error-handling paths; the issue does not name files, entry points, or tests. Read how current type-related failures are surfaced and establish the expected detail in the error messages. Done means core types are covered and errors provide the agreed descriptive context, with relevant tests passing.

Written by the indexing model from the issue text.