v0.4.0

Minor Changes

• 679d00f: HeadlessRunner caps error-controlled steps at 2 ms while the circuit is changing, and keeps the 10 ms H_MAX while it sits still. Backward Euler lags an RC curve by about half a step, and the local error tolerance alone let slow RC timing run at 10 ms steps: a textbook 555 astable (4.7 kohm, 10 kohm, 10 uF) ran 1.5% slow at 1x speed and 2.2% at 4x, and its period wandered by ±0.6 ms from cycle to cycle. With the cap it runs 0.6% slow with a ±0.04 ms spread. "Changing" means the last error estimate, extrapolated to a full 10 ms step (local error scales as h^(order+1)), would spend more than 10% of the tolerance; a rejected step counts as changing. A flat 2 ms ceiling was measured and rejected: it cost static logic and memory boards 3-4x the solves for no accuracy gain. Fixed-step runs and MCU circuits are unaffected.
• 679d00f: Add step breakpoints: SimEngine.captureStepBreakpoints, SimEngine.takeStepBreakpoints() and the StepBreakpoint type. An NE555 switch happens inside a step() (the engine splits the step at the threshold crossing), but a consumer that only sees accepted states interpolates straight across the whole step, so a scope drew the 555 output edge as a ramp centred on the step, up to half a step from where it switched. With capture on, the engine records the state at the switch and, after a 10 ns backward-Euler guard sub-step, the post-switch state; a split clamped short of a crossing in the last 2% of the step carries on to one guard short of the step end so the switch still lands inside it. Breakpoints roll back with restoreState() (snapshots carry stepBreakpointCount) and clear on load(). Capture is off by default, and with it off every trajectory is unchanged; with it on, the guard sub-step is the only change to the numerics, independent of whether anything reads the breakpoints.