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)