Improve op amp convergence for differentiator circuits

[esaruoho]

Summary

• Fixes ideal op amp "freaking out" in differentiator circuits with fast edges (op amp freaking out in differentiator #82)
• Two changes to OpAmpElm.doStep():
  1. Voltage limiting: clamps differential input voltage change between iterations to prevent Newton-Raphson overshoot. With gain=100000, the linear region is only ~0.3mV wide — without limiting, the iteration bounces wildly between saturation states.
  2. Deterministic convergence escape: replaces the random getrand(4)==1 check with a deterministic subIterations > 20 condition. The random 25% chance of switching saturation direction introduced nondeterministic oscillation that destabilized converging iterations in high-gain circuits.

Test plan

• Build op amp differentiator with 1k resistor + capacitor + logic input pulse → verify stable output
• Verify the built-in differentiator example (amp-dfdx.txt) still works correctly
• Test Schmitt trigger circuit (amp-schmitt.txt) → verify hysteresis still works
• Test standard op amp circuits (inverting, non-inverting, integrator) for regression
• Verify lower-gain op amps are unaffected

Fixes #82

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