📝 address PR feedback on spec

- Replace "descriptor" terminology with "directive" throughout
- Rename open() `name` parameter to `id` in spec
- Fix buffer diffing description: cells are copied, not swapped
- Add note that pointer hit detection requires render loop participation
- Reframe border-layout interaction as Clay's intended behavior
- Update WASM loading to reflect inlined base64 binary
- Move output buffer lifetime guarantee to normative section
- Clarify pack() is internal, validate() is public API

Author: cowboyd