feat(runtime): add streaming support to LlmDriver trait

- Add StreamChunk and StreamEvent types for Tauri event emission
- Add stream() method to LlmDriver trait with async-stream
- Implement Anthropic streaming with SSE parsing
- Implement OpenAI streaming with SSE parsing
- Add placeholder stream() for Gemini and Local drivers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-runtime/Cargo.toml

@@ -14,6 +14,7 @@ zclaw-memory = { workspace = true }
 tokio = { workspace = true }
 tokio-stream = { workspace = true }
 futures = { workspace = true }
+async-stream = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 thiserror = { workspace = true }