TypeScript Type Fixer and Strictness Upgrade

Fix all TypeScript errors in this file and improve type safety:
1. Replace 'any' types with proper specific types
2. Add return type annotations to all functions
3. Use type narrowing (typeof, instanceof, in, discriminated unions) instead of type assertions
4. Replace non-null assertions (!) with proper null checks
5. Add generics where functions work with multiple types
6. Use 'as const' for literal type inference where appropriate
7. Fix strict null checks — handle undefined/null properly
8. Use Record<K,V> instead of {[key: string]: any}
9. Export types that consumers of this module need
10. Use utility types (Partial, Required, Pick, Omit) appropriately

Explain each type decision briefly in comments when the type is non-obvious.
If a type requires a new interface/type, define it in the same file or suggest where it should live.