v0.4.0 — Mittelmann parity with MA57 + auto-armed cascade-break

Headline

feral is now at parity with MA57 on the 47-problem Mittelmann NLP
benchmark panel (Ipopt 3.14.20 + --linear_solver=feral vs
--linear_solver=ma57, 600 s wall timeout each):

solver	solve-rate	geomean wall (37 both-solved)	aggregate wall
MA57	39/47	1.000	3166 s
feral	39/47	0.960	1699 s (54%)

Two changes closed the gap: an auto-armed cascade-break default
in the C-ABI and a tightened MC64 routing gate. Headline rescues
on Mittelmann: marine_1600 -96%, clnlbeam -87%, robot_1600 -79%,
corkscrw -70%, dtoc2 rescued from timeout.

Highlights

• Auto-armed cascade-break in feral_new (Ipopt C-ABI). Arms
  Solver::with_auto_cascade_break(0.05) by default, disable with
  FERAL_AUTO_CB_BETA=0. (#37, #38)
• Scaling routing tightened. ScalingStrategy::Auto now requires
  a dense-arrow head (max_col_nnz > 32) in addition to the
  diag-only mass threshold, fixing a 1-D-PDE-KKT regression. (#68)
• MA57-style static-pivot perturbation knob + non-finite input
  validation + closed-form 2x2 eigenvalue inertia classifier +
  stale-MC64-cache invalidation. (#38)
• mittelmann_ipopt benchmark harness — runs Ipopt+feral and
  Ipopt+MA57 over the 47-problem NLP panel and produces a side-by-
  side report.
• Python bindings (feral-solver on PyPI) now at 0.4.0 with
  full IPM helper (feral.ipm.KktSolver), scipy.sparse adapters,
  and JAX interop.
• Ipopt integration documented end-to-end in the README, with the
  feral-ipopt-shim subproject providing one-shot
  make ipopt && make hs071.

See CHANGELOG.md
for the full per-PR release notes (auto-CB, scaling routing,
diagnostic tracers, env-knob reference, parity panel notes,
oracle-consensus correctness gate, and v0.4.0 housekeeping).

Install

pip install feral-solver         # Python wheels (CPython 3.10+)
cargo add feral                  # Rust crate

Compiling FERAL into Ipopt

See the "Using FERAL inside Ipopt" section of the README — one-shot
recipe is cd feral-ipopt-shim && make ipopt && make hs071.