feat(internal): add Cholesky factorization helper for SPD matrices

Hand-rolled `A = L · L^T` factorization plus forward/backward triangular
solves, used by the upcoming Bayesian Optimization implementation for
the GP posterior. Same f64 row-major Vec<Vec<f64>> interface as the
existing Jacobi eigen helper so we don't pull in nalgebra for one
algorithm.

Returns Err on non-positive-definite input (a small jitter is the
typical caller-side fix). Tested against the standard 2x2 case, the
3x3 known-result case, A·x = b round-trip, and the SPD-failure case.

src/internal/mod.rs

@@ -1,3 +1,4 @@
 //! Internal helpers used by built-in algorithms but not part of the public API.
 
+pub(crate) mod cholesky;
 pub(crate) mod eigen;