feat: expose exact determinant value (det_exact)

[acgetchell]

Summary

Expose the internal bareiss_det() function as a public API that returns the exact determinant as a BigRational, not just its sign.

Current State

det_sign_exact() returns only the sign (i8) of the determinant. Internally, it calls bareiss_det() which computes the full exact BigRational determinant, but this value is discarded after extracting the sign.

Proposed Changes

Add a public method:

pub fn det_exact(&self) -> Result<BigRational, LaError>

This would:

1. Validate entries are finite (same as det_sign_exact)
2. Run the Bareiss algorithm in BigRational arithmetic
3. Return the full exact determinant value

Optionally, also provide a convenience method that converts the result to f64:

pub fn det_exact_f64(&self) -> Result<f64, LaError>

Benefits

• Exact volume computation: Consumers like delaunay compute simplex volumes via det(Gram matrix). An exact determinant value would let them distinguish truly-degenerate simplices from near-degenerate ones.
• Minimal implementation effort: bareiss_det() already exists and is tested; this is primarily an API exposure change.
• Composable: Callers who need both the sign and value can use det_exact() instead of calling det_sign_exact() and then separately computing the value.

Implementation Notes

• Gate behind #[cfg(feature = "exact")]
• BigRational would become part of the public API surface — consider re-exporting it or documenting the num-rational dependency
• The fast f64 filter from det_sign_exact could optionally be used to return an exact f64 result when the error bound is small enough (i.e., det_direct() is already exact to full precision)

Co-Authored-By: Oz oz-agent@warp.dev