Blocks fails to load reliably on slow networks

In a school setting with a less than ideal network, it very often takes students several reload attempts before code.world/blocks will load for them. Not sure if this is some kind of race condition, or just a consequence of the size of the generated JavaScript in the `funblocks-client` package.

Whatever the case, this is a serious problem. It not only takes 5 to 10 minutes away from each class session, but far worse, it leaves students feeling like can't use the site without adults there ready to handle the problems.

低速なネットワークで code.world/blocks の読み込み失敗を再現し、funblocks-client パッケージと生成された JavaScript を調査します。その挙動が競合状態によるものか、ペイロードサイズの問題によるものかを特定し、低速なネットワークで繰り返しテストして検証できる信頼性の高い読み込み結果を定義します。

索引モデルが issue の本文から書いたものです。