Add time-aware PID control method (fixes #35)

[chantakan]

Summary

Adds next_control_output_with_dt(measurement, dt) method to handle time intervals properly in PID calculations, resolving the mathematical inaccuracies mentioned in #35.

Problem Solved

Previously, the PID controller ignored actual time intervals (dt) when calculating:

• Integral term: Was accumulating error * ki instead of error * ki * dt
• Derivative term: Was calculating measurement_change * kd instead of (measurement_change / dt) * kd

This led to inconsistent behavior when:

• Update intervals changed
• Sensor sampling was irregular
• Real-time constraints caused timing variations

Solution

New Method

• next_control_output_with_dt(measurement: T, dt: T) -> ControlOutput<T>
• Mathematically accurate integral: ∫e(t)dt ≈ Σ(error × dt)
• Mathematically accurate derivative: de/dt = Δmeasurement / dt

Backwards Compatibility

• Existing next_control_output() unchanged - now a thin wrapper calling new method with dt=1.0
• All existing tests pass
• No breaking changes to public API

Testing

Added comprehensive test coverage:

• integral_with_dt: Verifies proper time-weighted integration
• derivative_with_dt: Verifies proper rate-of-change calculation
• backwards_compatibility: Ensures existing behavior unchanged
• irregular_intervals: Tests real-world irregular timing scenarios
• All 14 tests pass (13 existing + 4 new)