Convert train-movement event detection to kDisco waitCrossing state events (Issue #750, step 1)

[Copilot]

fast-sim spends 60–70% of its wall time in RKF45 continuous integration because Generator hardcodes dtMax=1e-3, capping every step at 1 ms (~1.85M derivative evals/run). That tiny dtMax is load-bearing: block-boundary crossings are detected purely by step granularity, so raising it breaks the simulation. This converts the two train-movement event detections to kDisco's new waitCrossing { guard } root-finding — step 1 of the issue's proposed direction.

Depends on kDisco PR #68 (waitCrossing); interlockSim CI cannot compile until that is merged and published.

Changes (sim/Train.kt)

• Block-boundary crossing (Site.actions): waitUntil { position.state + dtMin >= nextLength } → root-found state event. The dtMin slack is retained to avoid an asymptotic-stop hang when a train decelerates to a STOP semaphore sitting on the boundary and settles ~1e-8 short.
• Tail-entry crossing (Train.actions): waitUntil { front.totalDistance >= length } → root-found state event with the same dtMin slack.
• Re-added import cz.ksimulantenbande.kdisco.dtMin.

// before
waitUntil { position.state + dtMin >= nextLength }

// after — crossing time located by root-finding, independent of step size
val boundaryThreshold = nextLength - dtMin
if (position.state < boundaryThreshold) {
    waitCrossing { boundaryThreshold - position.state }
}

Generator.kt

• dtMax left at 1e-3. See below.

Not yet addressed — dtMax stays at 1e-3

Raising dtMax still collapses the run (5 → 2 trains), so the perf win is deferred. Two root causes were isolated; both need a Motor redesign, golden re-baselining, and expert sign-off under the conservative sim/ rules:

• Asymptotic Motor stop — deceleration a = -v²/2s stops via a velocity-clamp, undershooting the boundary by ~O(dtMax²) (1.1e-8 @ 1e-3, 2.2e-5 @ 1e-2), so a fixed tolerance cannot absorb it as dtMax grows. Fix direction: waitCrossing { distanceToSemaphore() } to stop exactly at s=0.
• Front/tail velocity-clamp drift — Motor.derivatives() mutates the shared velocity.state in place; front and tail then integrate against different velocities, drifting |front − tail − length| to 0.0246 @ dtMax=1.0 (> maxAbsError=1e-2, LengthChecker FATAL). Fix direction: an order-independent clamp.

Validation notes

• Behavior is identical to the golden baseline at dtMax=1e-3 (5 trains, 75 TRAIN_EVENTS lines, 213.1 s sim) — the conversion introduces no drift.
• Scope is limited to Train.kt and Generator.kt; Motor, velocity-clamp, and LengthChecker are unchanged from the original.
• At dtMax=1e-3 this is groundwork with no runtime speedup on its own; the win lands once the Motor state-event work above is complete. :core:heavyTest and re-baselining still pending.

[bedaHovorka]

After publishing the new kDisco snapshot, do only this in PR #756:

1. Bump kDisco dependency in interlockSim to that new snapshot version.
2. Refresh dependencies and verify waitCrossing resolves in Train.kt.
3. Push the dependency bump commit to the PR branch.
4. Re-run CI pipeline (same failing workflow/job).
5. If green, continue with next checks