feat: 引入 aether-runtime/cache/data/http/testkit 基础 crate，完善并发门控与审计系统

新增 crate：
- aether-runtime: 服务运行时基础设施（并发门控、分布式并发、指标、队列、优雅关闭、tracing）
- aether-cache: 通用 TTL 缓存与命名空间抽象
- aether-data: 数据访问层（PostgreSQL/Redis 后端、repository 模式）
- aether-http: HTTP 客户端封装（重试、配置）
- aether-testkit: 集成测试工具集（gateway/executor/hub/proxy fixture、等待、负载测试）

gateway 扩展：
- 引入 audit 模块（shadow 执行审计、决策链路追踪、请求审计 bundle）
- 引入 cache 模块（AuthContext 缓存、direct-plan bypass 缓存）
- 引入 data 模块（auth/candidates/config/usage/video_tasks 数据访问）
- 集成 ConcurrencyGate/DistributedConcurrencyGate 请求门控
- 新增本地 auth 拒绝、过载响应构建器
- 补充 control/auth_cache/video/concurrency 集成测试

aether-proxy 扩展：
- AppState 集成 stream_gate / distributed_stream_gate 并发门控
- 新增 ProxyAdmissionError 及准入拒绝流程
- stream_handler 补充门控饱和/不可用场景测试
- 配置与注册客户端逻辑完善

aether-hub 扩展：
- main.rs 引入运行时初始化、指标端点、健康检查
- local_relay 重构为 lib.rs 暴露公共接口

crates/aether-testkit/src/gateway.rs

@@ -0,0 +1,76 @@
+use aether_gateway::{build_router_with_state, AppState};
+use aether_runtime::DistributedConcurrencyGate;
+
+use crate::server::SpawnedServer;
+
+#[derive(Debug, Clone)]
+pub struct GatewayHarnessConfig {
+    pub upstream_base_url: String,
+    pub control_base_url: Option<String>,
+    pub executor_base_url: Option<String>,
+    pub max_in_flight_requests: Option<usize>,
+    pub distributed_request_gate: Option<DistributedConcurrencyGate>,
+}
+
+impl GatewayHarnessConfig {
+    pub fn new(upstream_base_url: impl Into<String>) -> Self {
+        Self {
+            upstream_base_url: upstream_base_url.into(),
+            control_base_url: None,
+            executor_base_url: None,
+            max_in_flight_requests: None,
+            distributed_request_gate: None,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct GatewayHarness {
+    server: SpawnedServer,
+}
+
+impl GatewayHarness {
+    pub async fn start(config: GatewayHarnessConfig) -> Result<Self, String> {
+        Self::start_with_server(config, None).await
+    }
+
+    pub async fn start_on_port(config: GatewayHarnessConfig, port: u16) -> Result<Self, String> {
+        Self::start_with_server(config, Some(port)).await
+    }
+
+    async fn start_with_server(
+        config: GatewayHarnessConfig,
+        port: Option<u16>,
+    ) -> Result<Self, String> {
+        let mut state = AppState::new_with_executor(
+            config.upstream_base_url,
+            config.control_base_url,
+            config.executor_base_url,
+        )
+        .map_err(|err| format!("failed to build gateway harness state: {err}"))?;
+        if let Some(limit) = config.max_in_flight_requests {
+            state = state.with_request_concurrency_limit(limit);
+        }
+        if let Some(gate) = config.distributed_request_gate {
+            state = state.with_distributed_request_concurrency_gate(gate);
+        }
+        let router = build_router_with_state(state);
+        let server = match port {
+            Some(port) => SpawnedServer::start_on_port(port, router)
+                .await
+                .map_err(|err| format!("failed to start gateway harness: {err}"))?,
+            None => SpawnedServer::start(router)
+                .await
+                .map_err(|err| format!("failed to start gateway harness: {err}"))?,
+        };
+        Ok(Self { server })
+    }
+
+    pub fn base_url(&self) -> &str {
+        self.server.base_url()
+    }
+
+    pub fn port(&self) -> u16 {
+        self.server.port()
+    }
+}