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/src/driver/mod.rs

@@ -3,10 +3,14 @@
 //! This module provides a unified interface for multiple LLM providers.
 
 use async_trait::async_trait;
-use serde::{Deserialize, Serialize};
+use futures::Stream;
 use secrecy::SecretString;
+use serde::{Deserialize, Serialize};
+use std::pin::Pin;
 use zclaw_types::Result;
 
+use crate::stream::StreamChunk;
+
 mod anthropic;
 mod openai;
 mod gemini;
@@ -26,6 +30,13 @@ pub trait LlmDriver: Send + Sync {
     /// Send a completion request
     async fn complete(&self, request: CompletionRequest) -> Result<CompletionResponse>;
 
+    /// Send a streaming completion request
+    /// Returns a stream of chunks
+    fn stream(
+        &self,
+        request: CompletionRequest,
+    ) -> Pin<Box<dyn Stream<Item = Result<StreamChunk>> + Send + '_>>;
+
     /// Check if the driver is properly configured
     fn is_configured(&self) -> bool;
 }