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/scheduler/mod.rs

@@ -0,0 +1,28 @@
+mod candidate;
+mod failover;
+mod health;
+mod route;
+
+pub(crate) use candidate::{
+    list_selectable_candidates,
+    list_selectable_candidates_for_required_capability_without_requested_model,
+    read_cached_scheduler_affinity_target, read_minimal_candidate_selection,
+    GatewayMinimalCandidateSelectionCandidate,
+};
+pub(crate) use failover::{
+    resolve_core_stream_direct_finalize_report_kind,
+    resolve_core_stream_error_finalize_report_kind, resolve_core_sync_error_finalize_report_kind,
+    should_fallback_to_control_stream, should_fallback_to_control_sync,
+    should_finalize_sync_response, should_retry_next_local_candidate_stream,
+    should_retry_next_local_candidate_sync,
+};
+pub(crate) use health::{
+    count_recent_rpm_requests_for_provider_key, count_recent_rpm_requests_for_provider_key_since,
+    is_provider_key_circuit_open, provider_key_health_score, provider_key_rpm_allows_request_since,
+    PROVIDER_KEY_RPM_WINDOW_SECS,
+};
+pub(crate) use route::{
+    is_matching_stream_request, resolve_execution_runtime_stream_plan_kind,
+    resolve_execution_runtime_sync_plan_kind, supports_stream_scheduler_decision_kind,
+    supports_sync_scheduler_decision_kind,
+};