Add LDLt factorization for HMatrix and use it in the local DDM solvers

[PierreMarchand20]

Bunch-Kaufman LDLt for symmetric (real or complex) and Hermitian matrices, dense and hierarchical, selected by a symmetry of 'S' or 'H' (real 'H' falls back to the symmetric routines).

Dense (matrix/linalg/factorization.hpp):

• ldlt_factorization/ldlt_solve wrapping sytrf/sytrs and hetrf/hetrs.
• triangular_ldlt_matrix_matrix_solve: solve with the unit L/U, Bunch-Kaufman permutations included, transa in N/T/C like trsm.
• apply_ldlt_diagonal: D or D^-1 on the left or right, the 2x2 inverse in sytrs/hetrs's scaled form. A full solve is L^-1, D^-1, L^-T.
• Blas::geru/gerc (BLAS has no plain complex ger).

HMatrix:

• Block-recursive ldlt_factorization storing the unit L in place and D at the dense diagonal leaves: W = A21 L11^-T, L21 = W D1^-1, A22 -= L21 W^T. UPLO='U' eliminates the last cluster first.
• ldlt_solve, internal_triangular_ldlt_hmatrix_{hmatrix,matrix,lrmat}_solve and apply_ldlt_diagonal.hpp (keeps the compression of the blocks D is applied to, densifying with a warning only below the dense leaves).

Local DDM solvers:

• is_positive_definite option (default false) in LocalHMatrixSolver, LocalHMatrixPlusOverlapSolver, make_DDM_solver_w_custom_local_solver and DDMSolverBuilder. Symmetric/Hermitian local HMatrices now use LDLt unless declared positive definite, since Cholesky silently gave a wrong factorization for indefinite matrices. Complex symmetric always uses LDLt.

Fixes:

• HMatrix cholesky_factorization's symmetry check compared UPLO and could never trigger; LU/Cholesky checks now live in the public functions dispatching on the execution policy.
• potrf and sytrf/hetrf failures (info > 0) are now reported.

Tests: dense and HMatrix LDLt factorization and triangular solves, including 2x2 pivots, and Test_local_hmatrix_solvers checking both local solvers against the assembled system.

[codecov]

Codecov Report

❌ Patch coverage is 99.91857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.30%. Comparing base (658d8f0) to head (990ccde).
⚠️ Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
...x/linalg/triangular_ldlt_hmatrix_hmatrix_solve.hpp	99.04%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
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+ Coverage   97.19%   98.30%   +1.10%     
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  Files          80      188     +108     
  Lines        5132    11194    +6062     
==========================================
+ Hits         4988    11004    +6016     
- Misses        144      190      +46     

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