Fix #10385 (at least the few cases I've seen recently, I'll re-check if it still happens afterwards)

So apparently even 20km isn't always enough to add any amount of delay, because sometimes we barely have the traction to maintain the current speed. Ideally we should remove that check altogether, but it would still cause performance issues (repeatedly computing margins over potentially thousands of kilometers would certainly cause timeouts). We should look into #7136 for a proper solution.

EDIT: this version is better but it's still not enough. I've found a case with a 300km long allowance section that was still impossible. We may need to rework some stuff.

So there's two extra fixes:

1. Small bugfix on how we compute the "slowest envelope": we'd stop early if the max speed envelope ended with a speedup (we'd "intersect" right away)
2. In post-processing, prefill the fixed time point list with the end of every engineering allowance. This avoids some unnecessary and constraining extra fixed points at the wrong places.