docs: strengthen LDLT symmetry precondition documentation

[acgetchell]

Summary

The LDLT factorization assumes the input matrix is symmetric, but this precondition is only enforced via debug_assert_symmetric in debug builds. In release, an asymmetric input silently produces wrong results. The documentation should make this contract much more prominent.

Current State

• Ldlt::factor calls debug_assert_symmetric(&a) only under #[cfg(debug_assertions)]
• The doc comment on Matrix::ldlt says "intended for symmetric positive definite (SPD) and positive semi-definite (PSD) matrices" but doesn't warn about undefined behavior for asymmetric inputs
• The struct-level doc says "It assumes the input matrix is symmetric and (numerically) SPD/PSD" but this is easy to miss

Proposed Changes

1. Add a prominent # Preconditions or # Correctness section to Matrix::ldlt documentation explicitly stating that the input must be symmetric, and that violation produces silently wrong results in release builds
2. Consider adding a brief note that symmetry is checked only in debug builds (debug_assert)
3. Optionally, add a cheap spot-check in release builds (e.g., check a[0][1] == a[1][0] and a[0][D-1] == a[D-1][0] for D≥2) as a lightweight guard. This would be O(1) rather than the current O(D²) full check.

Benefits

• Users are clearly warned about the symmetry requirement
• Follows Rust documentation conventions for preconditions (similar to how sort_unstable documents its comparison contract)