refactor: 移除独立 hub/proxy/executor/gateway crate，统一为 gateway tunnel 架构

- 删除 aether-hub、aether-proxy 独立项目及其 Dockerfile/配置
- 删除 crates/aether-executor 和 crates/aether-gateway 全部模块
- 新增 apps/ 目录作为应用入口
- 将 hub 概念重构为 gateway tunnel transport
- 将 executor 重构为 execution runtime
- 新增 tunnel.rs 合约定义和 testkit tunnel/execution_runtime 模块
- 更新 Python 服务层和测试适配新架构命名

apps/aether-gateway/src/execution_runtime/ndjson.rs

@@ -0,0 +1,41 @@
+use std::io::Error as IoError;
+
+use aether_contracts::StreamFrame;
+use axum::body::Bytes;
+
+use crate::gateway::GatewayError;
+
+pub(crate) fn encode_stream_frame_ndjson(frame: &StreamFrame) -> Result<Bytes, IoError> {
+    let mut raw = serde_json::to_vec(frame).map_err(|err| IoError::other(err.to_string()))?;
+    raw.push(b'\n');
+    Ok(Bytes::from(raw))
+}
+
+pub(crate) fn decode_stream_frame_ndjson(line: &[u8]) -> Result<StreamFrame, GatewayError> {
+    serde_json::from_slice(line).map_err(|err| GatewayError::Internal(err.to_string()))
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::BTreeMap;
+
+    use aether_contracts::{StreamFramePayload, StreamFrameType};
+
+    use super::{decode_stream_frame_ndjson, encode_stream_frame_ndjson};
+
+    #[test]
+    fn ndjson_round_trip_preserves_frame() {
+        let frame = aether_contracts::StreamFrame {
+            frame_type: StreamFrameType::Headers,
+            payload: StreamFramePayload::Headers {
+                status_code: 200,
+                headers: BTreeMap::from([("content-type".into(), "text/event-stream".into())]),
+            },
+        };
+
+        let raw = encode_stream_frame_ndjson(&frame).expect("frame should encode");
+        let decoded =
+            decode_stream_frame_ndjson(raw.trim_ascii_end()).expect("frame should decode");
+        assert_eq!(decoded, frame);
+    }
+}