vfs: restore Version/Instance/SetInstance/Shutdown in cloud-free instance.go

These instance-lifecycle accessors lived in the cloud-coupled mount.go (removed
in v0.4.2). They are vfs-library surface, not a cloud concern — relocated here so
the standalone daemon, tests, and embedders keep the API while vfs stays
cloud-free. Fixes the v0.4.2 API regression.

instance.go

@@ -0,0 +1,51 @@
+// Instance lifecycle + version for the VFS package. Cloud-free: the
+// HIP-0106 mount adapter (which wires these into a zip.App) lives in
+// cloud/mounts/vfs, but the singleton instance and its accessors are a
+// vfs-library concern and stay here so the standalone daemon, tests,
+// and any embedder can attach/read a backend without importing cloud.
+
+package vfs
+
+import (
+	"context"
+	"sync"
+)
+
+// Version is overridden at build time via -ldflags
+// "-X github.com/hanzoai/vfs.Version=...".
+var Version = "dev"
+
+// mountedInstance is the VFS handle the HTTP/data-plane routes operate
+// against. nil until SetInstance() is called. Guarded by mu.
+var (
+	mu              sync.RWMutex
+	mountedInstance *VFS
+)
+
+// SetInstance attaches a VFS handle for the mounted routes. Calling this
+// with nil disables the data-plane routes. Idempotent.
+func SetInstance(v *VFS) {
+	mu.Lock()
+	defer mu.Unlock()
+	mountedInstance = v
+}
+
+// Instance returns the currently attached VFS handle (or nil).
+func Instance() *VFS {
+	mu.RLock()
+	defer mu.RUnlock()
+	return mountedInstance
+}
+
+// Shutdown drains the attached VFS instance (closing its backend).
+// Idempotent. Safe to call when no instance was attached.
+func Shutdown(_ context.Context) error {
+	mu.Lock()
+	defer mu.Unlock()
+	if mountedInstance == nil {
+		return nil
+	}
+	err := mountedInstance.Close()
+	mountedInstance = nil
+	return err
+}