chore(prototype): per-tick trace after rebuild, multilane only; record two-way result

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

docs/research/vehicle-probe.md

@@ -75,3 +75,8 @@ Scanned: the 9 mods in `C:\Users\Public\mod.io\10640\mods\` (e.g. `devers_1`, `m
   - Likely cause (human's hypothesis): the vehicles' `MovePath` refers to lanes of the removed segment, so the engine must re-attach them to the new segment and re-plan, and a vehicle can land in a different lane.
 - **Debug `MovePath` exchange is unusable from engine state**: `api.cmd.sendCommand(api.cmd.debug.makeComponentExchangeCmd(v, movePath))` fails with `Error decoding argument #3` even **without** a callback. So the earlier error wasn't caused by the callback. The only untried route is the GUI thread.
 - Not yet tested: a rebuild on a two-way (one lane per direction) road.
+
+## Rebuild on a two-way road (run 2026-10-05 ~21:12 UTC)
+
+- 3 `replaceSegment` rebuilds on `country_new_small` (one lane per direction), segments 30213, 29776 and 30215, follower 25.8 m behind leader. **No effect**: the next snapshot had the same gap (22.6/48.4 → 49.3/75.1), both vehicles stayed in lane 2, same speeds, same order for 10+ snapshots. Rebuilding does not move vehicles past each other on single-lane roads.
+- How the multilane lane change happens is still uncaught: snapshots every 2 s only show before (both lane 4) and after (the leader in lane 3 two segments later). The probe now traces the two vehicles on every update for ~30 s after a rebuild (`TRACE` lines), and rebuilds only multilane segments.