fix(endgames): NaN is a failure, never Converged; NaN-aware security valve

[ofloveandhate]

Closes the NaN-blindness hole in both endgames, found during the PSEG audit (follow-up to #353).

The hole

Every IEEE comparison against NaN is false. PSEG's run loop converges on approx_error > FinalTolerance() going false — so a NaN extrapolation exited the loop down the success path, reporting Converged with a poisoned answer. Cauchy's inverse-polarity loop (while(true) + explicit error < tol check) instead slogged pointlessly to MinTrackTime doing NaN arithmetic. The security valve shared the blindness: norm > max_norm is false for a NaN dehomogenized norm, disarming the divergence bailout for exactly the paths most likely at infinity (a NaN dehom norm means the homogenizing coordinate vanished). There was not a single isnan anywhere in endgames/ or trackers/.

The subtlety worth knowing

complex_mp equality does not follow IEEE NaN semantics: a NaN complex_mp compares EQUAL to itself (real_mp behaves correctly). So the standard self-inequality trick (z != z) — and Eigen's own hasNaN() — silently miss NaN at exactly the mp types. Detection must be component-wise isnan on real and imaginary parts.

Changes

• bertini::ContainsNaN in eigen_extensions.hpp (next to IsEmpty, usable everywhere): component-wise isnan over any complex-valued Eigen object, with the mp caveat documented at the definition.
• Both extrapolation functions (ComputeApproximationOfXAtT0, ComputeCauchyApproximationOfXAtT0) return FailedToConverge on a NaN result; the run loops already bail on any non-Success extrapolation code, so no loop restructuring.
• EndgameBase::BeyondSecurityMaxNorm: NaN counts as beyond max_norm; all four valve sites (2 PSEG, 2 Cauchy) route through it.
• ComputeCycleNumber defaults its selection to 1 before the candidate loop: with poisoned samples no candidate ever wins (NaN comparisons), and a fresh endgame carried cycle number 0 into TransformToSPlane and threw. Behavior-neutral in normal operation (candidate 1 always beats highest()).
• Named regression tests, one per endgame, injecting a NaN sample and asserting a failure code. The tests were written red-first: the first version used z != z and passed nowhere on mp — which is how the complex_mp equality wrinkle was discovered.

Tests

411 endgame cases green, full ctest battery green, C++ doclint clean.

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