fix(pool): gate snapshot-fork trigger_snapshot to unix (Windows build)

warm_pool::trigger_snapshot connects to libkrun's snapshot trigger socket via
tokio::net::UnixStream, which does not exist on Windows (E0433). Snapshot-fork
is a Linux/KVM feature, so gate the real impl to #[cfg(unix)] and add a
#[cfg(not(unix))] stub that errors — mirroring the existing cfg(not(windows))
pattern in the pool CLI. Fixes the v2.1.0 Build Windows (WHPX) failure.

Author: Roy Lin

src/runtime/src/pool/warm_pool.rs

@@ -500,6 +500,10 @@ impl WarmPool {
 
     /// Send a `snapshot <state>` request to libkrun's per-template trigger socket and
     /// wait for the `ok` reply (the socket appears once the template's vCPUs run).
+    ///
+    /// Snapshot-fork is a Linux/KVM (Unix) feature; on non-Unix hosts the trigger
+    /// socket does not exist, so this is unavailable (see the `not(unix)` stub).
+    #[cfg(unix)]
     async fn trigger_snapshot(sock: &std::path::Path, state_file: &std::path::Path) -> Result<()> {
         use tokio::io::{AsyncReadExt, AsyncWriteExt};
         // The socket is bound by libkrun after the guest starts; poll briefly.
@@ -534,6 +538,16 @@ impl WarmPool {
         }
     }
 
+    /// Non-Unix stub: snapshot-fork relies on libkrun's Unix trigger socket and KVM
+    /// state save/restore, neither of which exist on Windows. `--snapshot-fork` is
+    /// Linux/KVM-only, so this path is never reached there in practice.
+    #[cfg(not(unix))]
+    async fn trigger_snapshot(_sock: &std::path::Path, _state_file: &std::path::Path) -> Result<()> {
+        Err(BoxError::PoolError(
+            "snapshot-fork is only supported on Linux/KVM hosts".to_string(),
+        ))
+    }
+
     /// Fill the pool to the minimum idle count.
     async fn fill_to_min(&self) {
         let current = self.idle.lock().await.len();