fix(examples): streams example needs code fixes

## Problem

The `streams` example doesn't compile:
- Uses `#[entity(streams)]` feature
- Needs entity-core and futures dependencies
- Code needs updates to match current macro API

## Tasks

- [ ] Verify streams feature is implemented
- [ ] Add entity-core and futures dependencies
- [ ] Ensure example compiles and runs

## Related

Part of examples restructuring (#66)