test: add LDLT NonFinite coverage tests; update README example

- Add tests for L-multiplier overflow and trailing submatrix overflow
  during LDLT factorization
- Add test for forward substitution overflow in LDLT solve_vec
- Update README det_sign_exact example for Result API (.unwrap())

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

Author: acgetchell

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Return Result from det_sign_exact, rename NonFinite field [`717d5cf`](https://github.com/acgetchell/la-stack/commit/717d5cf13af77764e8941004cdf2c153c260690f)
+
 ## [0.2.0] - 2026-03-07
 
 ### Added
@@ -46,6 +52,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Replace cspell with typos-cli, bump MSRV to 1.94.0 [`6db12be`](https://github.com/acgetchell/la-stack/commit/6db12bebdc332f93f9663c0c0be8e785ab449fa6)
 - Add changelog tooling (git-cliff + tag script) [`5475651`](https://github.com/acgetchell/la-stack/commit/547565142a374d6a7fed432dd6dbaa8b24cf98a1)
+- Release v0.2.0 [`922006d`](https://github.com/acgetchell/la-stack/commit/922006d4a6d905bdb66b7eb729e0cb4eca1da70a)
 
 ### Performance
 
@@ -138,6 +145,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add tarpaulin coverage upload [`7486dfd`](https://github.com/acgetchell/la-stack/commit/7486dfd54e16a6dbde41575c3f35a1acb65f57d2)
 
+[unreleased]: https://github.com/acgetchell/la-stack/compare/v0.2.0..HEAD
 [0.2.0]: https://github.com/acgetchell/la-stack/compare/v0.1.3..v0.2.0
 [0.1.3]: https://github.com/acgetchell/la-stack/compare/v0.1.2..v0.1.3
 [0.1.2]: https://github.com/acgetchell/la-stack/compare/v0.1.1..v0.1.2

README.md

@@ -153,7 +153,7 @@ let m = Matrix::<3>::from_rows([
     [4.0, 5.0, 6.0],
     [7.0, 8.0, 9.0],
 ]);
-assert_eq!(m.det_sign_exact(), 0); // exactly singular
+assert_eq!(m.det_sign_exact().unwrap(), 0); // exactly singular
 ```
 
 For D ≤ 4, a fast f64 filter (error-bounded `det_direct()`) resolves the sign

src/ldlt.rs

@@ -333,4 +333,37 @@ mod tests {
         let err = a.ldlt(DEFAULT_SINGULAR_TOL).unwrap_err();
         assert_eq!(err, LaError::NonFinite { col: 0 });
     }
+
+    #[test]
+    fn nonfinite_l_multiplier_overflow() {
+        // d = 1e-11 > tol, but l = 1e300 / 1e-11 = 1e311 overflows f64.
+        let a = Matrix::<2>::from_rows([[1e-11, 1e300], [1e300, 1.0]]);
+        let err = a.ldlt(DEFAULT_SINGULAR_TOL).unwrap_err();
+        assert_eq!(err, LaError::NonFinite { col: 0 });
+    }
+
+    #[test]
+    fn nonfinite_trailing_submatrix_overflow() {
+        // L multiplier is finite (1e200), but the rank-1 update
+        // (-1e200 * 1.0) * 1e200 + 1.0 overflows.
+        let a = Matrix::<2>::from_rows([[1.0, 1e200], [1e200, 1.0]]);
+        let err = a.ldlt(DEFAULT_SINGULAR_TOL).unwrap_err();
+        assert_eq!(err, LaError::NonFinite { col: 0 });
+    }
+
+    #[test]
+    fn nonfinite_solve_vec_forward_substitution_overflow() {
+        // SPD matrix with large L multiplier: L[1,0] = 1e153.
+        // Forward substitution overflows: y[1] = 0 - 1e153 * 1e156 = -inf.
+        let a = Matrix::<3>::from_rows([
+            [1.0, 1e153, 0.0],
+            [1e153, 1e306 + 1.0, 0.0],
+            [0.0, 0.0, 1.0],
+        ]);
+        let ldlt = a.ldlt(DEFAULT_SINGULAR_TOL).unwrap();
+
+        let b = Vector::<3>::new([1e156, 0.0, 0.0]);
+        let err = ldlt.solve_vec(b).unwrap_err();
+        assert_eq!(err, LaError::NonFinite { col: 1 });
+    }
 }