Fix navigation braille in no-unsafe builds

[venkatesh6114]

Summary

Fixes a panic in get_navigation_braille when MathCAT is compiled with the no-unsafe feature.

When the navigation focus is below the outer <math> element, get_navigation_braille creates a temporary document and wraps the focused node in a new <math> element. Previously, the copied node was allocated in the original document and then appended to the temporary document.

The default backend happens to tolerate this cross-document insertion, but the no-unsafe backend uses document-local indices and can panic with an out-of-bounds error.

Changes

• Add a no-unsafe-specific helper that recursively copies a MathML subtree into a specified destination document.
• Use the destination-document copy when wrapping the current navigation focus in a temporary <math> element.
• Preserve the existing copy_mathml path for builds without no-unsafe.
• Add a feature-gated regression test that:
  • Sets MathML containing a focused child node.
  • Moves the navigation focus to that node.
  • Calls get_navigation_braille.
  • Verifies that the expected LaTeX output is returned without a panic.

Compatibility

The behavior change is limited to builds compiled with the no-unsafe feature:

#[cfg(feature = "no-unsafe")]

Fixes #827

[github-actions]

Linux library size: 0.58 MiB (0.00%)

Revision	Release liblibmathcat.so
Base (95892ab)	0.58 MiB (611,568 bytes)
PR (b01ca08)	0.58 MiB (611,568 bytes)
Change	0 bytes (0.00%)

Built with default features, Rust 1.96.0, and Ubuntu 24.04. Workflow run.

[NSoiffer]

Thanks for spotting the bug and explaining what the problem is. I am committing a different fix (b13ec5a) that avoids the #[cfg(feature = "xxx")] usages and instead just adds the extra arg to the copy function. Ultimately, it is a little less code although changes are in a few more places. I used your test (without the feature flag so both versions are tested).