B1 (api-performance): wire hyper_ideal_hessian_block_fd_sym into newton_hyper_ideal
instead of the full-FD path — 33×/1166× faster on cathead/brezel, also fixes the
FD-step vs Newton-tol accuracy floor (N1 cross-fix).
V3 (input-validation): add isfinite check on all vertex coordinates in load_mesh;
NaN/Inf now throws runtime_error at the I/O boundary before poisoning the solver.
C1 (test-coverage): expose the internal ok flag via an optional bool* parameter
on solve_linear_system so double-solver failure is no longer invisible to callers.
+5 new tests (LoadMeshThrowsOnNaN/Inf, OkFlag_True*, OkFlag_NullPointerIsSafe).
282/282 tests pass.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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