Test hardening: edge cases + numerical-stability regressions #61

Fill the remaining test gaps before v0.1.0. Complements #15 (save/load + plot_components).

Tasks

• Single-channel and single-sample edge-case fits (minimal real slices, not synthetic) that assert graceful behavior (fit or a clear error, no silent NaN).
• Numerical-stability regression tests exercising the Fortran bounds (mincond, minlog, maxdble, mineig): inputs near-singular / extreme-scale should hit the clamps deterministically.
• Wire these into CI (they are fast; keep the Fortran-binary parity tests behind the slow marker).

Notes

NO-MOCK: correctness tests use real sample EEG + the Fortran binary. Edge-shape tests may use minimal real slices. See #15 for the save/load + plot_components half of this work.