v0.8.0 — feral default

[0.8.0] - 2026-05-06

BREAKING: the default sparse linear solver has changed from rmumps to
feral, a pure-Rust multifrontal LDLᵀ solver with Bunch-Kaufman
1×1/2×2 pivoting, certified inertia, MC64 scaling, and AMD/METIS ordering.
rmumps remains available behind the opt-in rmumps feature for A/B
comparison and regression hunting.

Changed

• Default sparse linear solver: feral (replaces rmumps).
  • default-features = ["feral", "faer"] (was ["rmumps", "faer"]).
  • Build with the legacy backend via
    cargo build --no-default-features --features "rmumps faer".
  • HS suite parity: 118/120 Optimal under both backends with zero status
    flips (HS116/HS374 fail under both). Geomean solve-time within 5%.
  • LinearSolverChoice::{Direct, Iterative, Hybrid} now route to
    FeralLdl / FeralIterativeMinres / FeralHybrid by default. Under
    the feral backend, Iterative reduces to Direct + iterative refinement
    (the full LDLᵀ as MINRES preconditioner converges in one step); a true
    incomplete-LDLᵀ + MINRES path is deferred to v0.9.

Added

• src/linear_solver/feral_direct.rs — FeralLdl wrapper with cached
  symbolic factorization, COO→CSC value scatter, MA27-style pivot-threshold
  escalation in increase_quality(), and ZeroPivotAction::ForceAccept so
  ripopt's existing inertia-correction loop drives perturbation.
• src/linear_solver/feral_iterative.rs, feral_hybrid.rs — companions
  for LinearSolverChoice::Iterative and Hybrid.
• tests/feral_solvers.rs — direct trait-surface tests for FeralLdl.