chore(prototype): remove segment rebuild and dead engine-side experiments

Rebuild confirmed ineffective on single-lane roads (log + in game). Engine
side is logging only again; the GUI-thread position flip is the one
experiment left.

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

docs/research/vehicle-probe.md

@@ -80,3 +80,7 @@ Scanned: the 9 mods in `C:\Users\Public\mod.io\10640\mods\` (e.g. `devers_1`, `m
 
 - 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.
+
+## Segment rebuild dropped (2026-10-05)
+
+The human confirmed in game what the log showed: rebuilding the segment under a held-back vehicle does not get it past the blocker on a single-lane road. The rebuild code is removed from the probe (it's still in git history, last in commit 5d3a254). The lane split after rebuilds on multilane roads (see above) is left as an observation for the multilane ticket, not pursued here.