Additional ext4-backed mount point for high-throughput I/O caching (besides `/kaggle/working`)

## 🚀 Feature

Provide an additional mount point backed by a physical device (`ext4`), separate from `/kaggle/working`, to use as a high-throughput I/O cache (e.g., `/tmp`).

## Motivation

* The container root (`/`) appears to be provided via an overlay filesystem; write/metadata performance on overlay can be slower or less predictable than on a host-backed volume such as `/kaggle/working`.
* Some applications default to `/tmp` for I/O-intensive caches; on overlay this default can become an I/O bottleneck.
* `/kaggle/working` is relatively fast and can be an alternatives to `/tmp`, but its capacity is limited to \~20 GB per session, which is sometimes insufficient to host checkpoint of LLMs (\~100GB).
* An additional fast endpoint for caching has the potential to better utilize the instance’s compute resources (e.g., T4, P100) that are often bottlenecked by disk I/O.

## Additional context

I benchmarked sequential I/O and observed both writes and reads to be slower on `/tmp` (overlay) than on `/kaggle/working` (host-backed). Write throughput on `/tmp` is especially unstable; it sometimes gets ~5–10× lower in my tests.

* [🕵️ I/O Performance Test in Kaggle Kernel](https://www.kaggle.com/code/tatamikenn/i-o-performance-test-in-kaglle-kernel)

まず、issue の I/O ベンチマークと既存のコンテナマウント設定を確認してください。レポートでは実装ファイルやテストは指定されていません。追加の物理デバイス backed の ext4 マウントを /kaggle/working とは別に定義し、その後、既存のマウント動作を変更せずに、より大きく高速なキャッシュパスを提供することを検証してください。

索引モデルが issue の本文から書いたものです。