feat: 引入 Rust executor/gateway sidecar 及 Python 侧双后端适配

- 新增 Rust workspace crates: aether-contracts, aether-executor, aether-gateway
- aether-executor: 支持 Unix Socket/TCP 双传输模式，处理同步/流式上游请求
- aether-gateway: 作为本地主入口代理，集成 /api/internal/gateway/resolve 认证预解析
- Python 侧新增 ExecutionPlan 契约和 RustExecutorClient，各 handler 支持
  executor_backend=rust 时将可序列化请求转发给 Rust executor 执行
- 重构 dev.sh 支持 executor/gateway 进程编排与生命周期管理
- 新增 internal gateway 路由，提供 resolve/passthrough 端点
- handler 层(chat/cli/video/endpoint_checker 等)全面适配 Rust executor 回退逻辑
- pipeline 层支持 trusted auth context 跳过重复认证
- 新增 Rust CI workflow 及对应测试用例

crates/aether-contracts/src/frame.rs

@@ -0,0 +1,58 @@
+use std::collections::BTreeMap;
+
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+use crate::{ExecutionError, ExecutionTelemetry};
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum StreamFrameType {
+    Headers,
+    Data,
+    Error,
+    Telemetry,
+    Eof,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(tag = "kind", rename_all = "snake_case")]
+pub enum StreamFramePayload {
+    Headers {
+        status_code: u16,
+        #[serde(default)]
+        headers: BTreeMap<String, String>,
+    },
+    Data {
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        chunk_b64: Option<String>,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        text: Option<String>,
+    },
+    Error {
+        error: ExecutionError,
+    },
+    Telemetry {
+        telemetry: ExecutionTelemetry,
+    },
+    Eof {
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        summary: Option<Value>,
+    },
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+pub struct StreamFrame {
+    #[serde(rename = "type")]
+    pub frame_type: StreamFrameType,
+    pub payload: StreamFramePayload,
+}
+
+impl StreamFrame {
+    pub fn eof() -> Self {
+        Self {
+            frame_type: StreamFrameType::Eof,
+            payload: StreamFramePayload::Eof { summary: None },
+        }
+    }
+}