sausage-vfs: review fixes (fallible growth, symlink flags, stream cancel, sized reads, timestamps, error kinds, linker shadowing)

API: Layer::stat(path, follow), Layer::readlink, Open::follow,
Stat::symlink and Entry::symlink. Process::idle documents the shutdown
order: abort, await the task, await idle, release locks.

vfs/src/process.rs

@@ -36,7 +36,10 @@ impl Process {
         .await
         .map_err(std::io::Error::other)?
     }
-    /// Wait for all outstanding system calls. The caller must prevent new operations.
+    /// Wait for all outstanding system calls.
+    /// First abort the guest task. Then await its JoinHandle, or drop its store.
+    /// Only then await idle before releasing locks. Abort alone does not stop a running poll.
+    /// This ordering also prevents later memory writes, which need no blocking guard.
     pub async fn idle(&self) {
         loop {
             let notified = self.0.idle.notified();