vfs/cache: Fix promoteToFast race with WriteCloser Close

promoteToFast read FileInfo.Size from Stat after the VFS lock was
released, while WriteCloser.Close updated Size on the same live
object. Concurrent Create/Open on overlapping keys tripped the race
detector.

Stat now returns a FileInfo snapshot, and promotion uses the
ReadAll length for the fast-tier Create size. Promotion stays
best-effort.

#21

vfs/types/types.go

@@ -27,6 +27,17 @@ func NewFileInfo(key string, size int64) *FileInfo {
 	}
 }
 
+// Clone returns a snapshot copy of fi. Stat returns Clone() so callers can
+// read Size and other fields without racing Close/Open mutations of the
+// in-map FileInfo.
+func (fi *FileInfo) Clone() *FileInfo {
+	if fi == nil {
+		return nil
+	}
+	cp := *fi
+	return &cp
+}
+
 // NewFileInfoFromOS creates a FileInfo from os.FileInfo
 func NewFileInfoFromOS(info os.FileInfo, key string) *FileInfo {
 	return &FileInfo{