fix: atomic writes for posts and users.toml

Write to a .tmp file and then File.rename into place, so a concurrent
read or a mid-write crash never sees a torn file.

lib/volumen/store.rb

@@ -37,7 +37,9 @@ module Volumen
     def save(post)
       target = post.path || default_path_for(post)
       FileUtils.mkdir_p(File.dirname(target))
-      File.write(target, post.to_file)
+      tmp = "#{target}.tmp"
+      File.write(tmp, post.to_file)
+      File.rename(tmp, target)
       post.path = target
       @all = nil
       post

lib/volumen/users.rb

@@ -118,7 +118,9 @@ module Volumen
 
     def persist(users)
       FileUtils.mkdir_p(File.dirname(@path))
-      File.write(@path, TomlRB.dump("users" => users.map { |user| user_hash(user) }))
+      tmp = "#{@path}.tmp"
+      File.write(tmp, TomlRB.dump("users" => users.map { |user| user_hash(user) }))
+      File.rename(tmp, @path)
     end
 
     def user_hash(user)