## Summary
* **What is the goal of this PR?** Fix a dangling pointer issue caused
by using `.c_str()` on a temporary `std::string`.
`basepath.substr()` creates a temporary `std::string`, and calling
`.c_str()` on it returns a pointer to its internal buffer (not a copy).
Since the temporary string is destroyed at the end of the full
expression, `folderName` ends up holding a dangling pointer, leading to
undefined behavior.
To solve this, we stores the result in a persistent `std::string`
object, ensuring the underlying buffer remains valid for the duration of
its use.
A similar pattern caused the behavior reported in
https://github.com/crosspoint-reader/crosspoint-reader/pull/728#issuecomment-3902529697
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? _**< NO >**_
dc6562a51c