fix: replace panicking unwrap() calls with descriptive errors in build.rs

[plasmaDestroyer]

Problem

In dynamic_linking() in build.rs, two .unwrap() calls can panic with an unhelpful "called unwrap() on None" message:

• file_stem().unwrap() panics if the path has no valid filename
• parent().unwrap() panics if the path has no parent directory

Fix

Replaced both with .expect() to get a clear, readable message explaining what went wrong instead of a panic.

Note

Also noticed a similar .unwrap() in the vcpkg_linking module when converting paths to UTF-8. This could potentially panic on Windows if FFmpeg is installed in a path with non-UTF-8 characters, though likely rare. Leaving it out of this PR to keep changes focused, can be addressed separately if needed.