Add fixed-point and stability diagnostic

[rickyjreyes]

Adds a NumPy-only scientific diagnostic for the current WaveLock evolution.

It measures:

• normalized fixed-point residuals at multiple checkpoints;
• denominator proximity to zero and one-step amplification;
• exact small-grid Jacobian eigenvalues and spectral radius;
• perturbation contraction against the unperturbed continuation;
• classification as numerical fixed point, evolving transient, uniform damped transient, or non-finite state.

The smoke profile is pytest-compatible and uses only NumPy. Larger standard and deep profiles can be selected with WAVELOCK_FIXED_POINT_PROFILE.

Local validation of the fast profile produced:

• terminal classification: uniform_damped_transient;
• residual: 2.000003e-05;
• relative spatial standard deviation: 2.822178e-05;
• Jacobian spectral radius: 0.99998;
• perturbation final gain after 500 steps: 0.163144.

This intentionally does not label the current 50-step terminal state as a fixed point unless the residual test is actually met.