fix(runtime): restore per-instance native thread configuration (#2658)

Restore per-instance Tokio executors for standard native instances,
honoring multi_thread and multi_thread_count with a two-worker minimum
in multi-thread mode. Preserve external execution for compact hosts.

Run construction, lifecycle operations, FFI data-plane tasks, and
hot-reloaded port forwards on the selected instance executor. Keep
shutdown cleanup running when its caller is cancelled, and release
owned executors without blocking their workers on runtime teardown.

Add runtime ownership, cancellation, cross-runtime connectivity, and
hot-reload regression coverage, and document the execution model.

Validation: all 45 PR checks passed, including linting, integration
tests, and cross-platform builds.

easytier/src/instance/composition.rs

@@ -31,7 +31,10 @@ use crate::{
     instance::runtime_host::NativeInstanceRuntimeHost,
 };
 
-use super::host::{NativeInstanceHost, native_instance_host};
+use super::{
+    host::{NativeInstanceHost, native_instance_host},
+    runtime_executor::NativeInstanceExecutor,
+};
 #[cfg(feature = "kcp")]
 use crate::gateway::kcp_proxy::KcpProxyService;
 #[cfg(feature = "quic")]
@@ -42,10 +45,11 @@ use easytier_core::gateway::proxy::wrapped_transport::WrappedTransportEngine;
 
 pub(crate) type NativeCoreInstance = CoreInstance<NativeInstanceHost>;
 
-pub(crate) fn compose_native_core_instance(
+pub(super) fn compose_native_core_instance(
     toml_config: TomlConfig,
     process_runtime: Arc<CoreProcessRuntime>,
     compact_runtime: bool,
+    executor: Option<NativeInstanceExecutor>,
 ) -> anyhow::Result<Arc<NativeCoreInstance>> {
     let host_config = if compact_runtime {
         compact_runtime_core_host_config()
@@ -58,7 +62,7 @@ pub(crate) fn compose_native_core_instance(
         &normalized,
         &host_config,
     ));
-    let runtime_host = NativeInstanceRuntimeHost::new(global_ctx.clone());
+    let runtime_host = NativeInstanceRuntimeHost::new_with_executor(global_ctx.clone(), executor);
     let mut adapters = runtime_core_host_adapters_with_packet_egress_and_config(
         global_ctx.clone(),
         process_runtime,