feat: surface degenerate covariance and NaN distance failures explicitly

[t-uda]

Motivation

Current behavior often propagates NaN values silently, which forces users to discover degeneracy indirectly in downstream code. Demo notebooks had to add manual np.isfinite(...) guards.

Scope

• Detect common failure cases around singular covariances and invalid tangency outputs.
• Introduce opt-in warnings or clearer exceptions without breaking current workflows abruptly.

Acceptance Criteria

• At least the most common degeneracy paths are surfaced with actionable messages.
• Existing users still have a compatible path for workflows that rely on NaN-based detection.
• Tests cover both the warning/validation path and backward-compatible behavior.

Out of Scope

• Redesigning the full numerical stability contract across all solvers.