feat(sse): add shared SSE codec module

model_gateway/src/routers/common/mod.rs

@@ -17,9 +17,12 @@
 //!   used by every router for transport-level retries. Has zero
 //!   coupling to the `Worker` trait — it lived in `worker/` for
 //!   historical reasons before this extraction.
+//! - [`sse`] — shared SSE codec (encoder + decoder) for streaming
+//!   responses to clients and parsing upstream SSE byte streams
 
 pub mod header_utils;
 pub mod mcp_utils;
 pub mod persistence_utils;
 pub mod retry;
+pub mod sse;
 pub mod worker_selection;