chore(prototype): keep street variant B only; record second run

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/research/vehicle-probe.md

@@ -98,3 +98,9 @@ The human confirmed in game what the log showed: rebuilding the segment under a
 - First in-game run (human, 2026-10-05): the template loads, the lanes are visibly offset as intended, and vehicles can drive on the passing lanes. **Town (non-player) cars sometimes choose the passing lane.** **A faster bus behind a slower vehicle does not switch to the empty lane**: the switching logic is missing.
 - Changes for the next run: passing lanes allow only `BUS`/`TRUCK` (keeps private `CAR` town traffic out), and the probe logs the road template per vehicle (`road=`), since lane indices 2/3 also mean ordinary lanes on `country_new_medium`.
 - Hypothesis for the switch: the engine only re-chooses lanes at segment boundaries (`GetNextEdgeTrafficPenaltyAndAlternatives`), so short segments give a blocked bus more chances to pick the passing lane. Fallback trigger: `replaceSegment` ahead of the pair (made the leader re-plan lanes on multilane roads).
+- Second run (2026-10-05 ~21:50 UTC, variant B, passing lanes BUS/TRUCK only). The human judged **variant B correct** (backward passing lane offset -2.5, forward +2.5), so variant A was removed. On the prototype road, lane 4 is the normal forward lane and lane 3 the forward passing lane.
+  - **An overtake happened**: fast 29251 (lane 4, 22.22 m/s) got past slow 29851 over segments 29707 → 30211 (side by side) → 30215 → 29708 → 29042 and stayed ahead. But the *slow* vehicle was in the passing lane (lane 3), and the passer went past in the normal lane.
+  - The engine treats both lanes as equal: some vehicles cruise in the passing lane almost the whole way (29796: 28/28 samples in lane 3, 27775: 29/29). Others mix (29251: 33 in lane 4, 14 in lane 3).
+  - Vehicles slowed to 9–10 m/s and briefly stopped on segment 27290 (presumably the end of the prototype stretch): a merge from two lanes into one.
+  - Visually the lane change is abrupt (human: "the lanes need a smoother transition"): the switch happens at the node, which has almost zero length on a straight, and the jump is about 5–6 m sideways.
+  - Next ideas: a lower speed limit on the passing lane (prefer the normal lane unless it's blocked), a smaller offset or transition segments for a smoother change.