refactor: centralize key-path resolution in operation-to-patches

Extracts a `resolveKeyPath` function that converts positional Slate paths
to key-based PT paths using the tree. Uses it in operation-to-patches.ts
to replace scattered inline key lookups.

This is the foundation for making operations and patches isomorphic —
when containers land, resolveKeyPath generalizes to walk nested container
fields, and the patch functions don't need to change.

- New: resolve-key-path.ts with resolveKeyPath() + KeyPath type
- Refactored: operation-to-patches.ts uses resolveKeyPath + path helpers
- New: 11 tests for resolveKeyPath covering blocks, children, objects, edge cases
- All 255 existing unit tests pass

Author: christianhg