Task 3.4: Cross-Runtime Communication Strategy

**Epic**: #314
**Phase**: Phase 3

### Description
Design cross-runtime communication (.NET ↔ WASM).

### Activities
1. Design IR serialization for WASM
2. Design type marshalling
3. Design async operation support
4. Create benchmarks

### Deliverables
- [ ] Serialization format spec
- [ ] Type marshalling guide
- [ ] Async operation design
- [ ] Performance benchmarks

**Effort**: 3 days