Document GPS routing research

docs/compaction-handoff.md

@@ -2,6 +2,28 @@
 
 Current date/time context: 2026-06-27, local timezone America/Chicago.
 
+## 2026-06-27 Routing Literature And Engine Research
+
+- Added `docs/gps-routing-research.md` as the durable summary of the GPS
+  routing research pass.
+- Main conclusion: stop treating "prefer highways" as the objective. It is a
+  useful proxy, especially for AutoDrive, but manual player GPS should optimize
+  momentum/corridor quality: low turn count, low short-segment churn, good
+  continuity, fewer awkward ramps/intersections, and later slope/airtime risk.
+- Production engines split edge cost from transition/turn cost. Mirror that at
+  the native solver relaxation surface instead of using another broad highway
+  multiplier. Prefer nonnegative additive penalties first so the vanilla
+  A*/Dijkstra-like assumptions stay sane.
+- The user explicitly allows external dependencies if they are justified.
+  Recommendation after research: do not link a full production engine yet.
+  Borrow OSRM/Valhalla/GraphHopper cost-model ideas now, and consider
+  RoutingKit or a tiny custom A* runner offline for graph/cost experiments.
+- Full in-game engine replacement remains feasible but expensive. It requires
+  extracting/owning the full drivable graph, mapping live start/target points,
+  computing the path, and converting the result back into Cyberpunk route
+  handles/records. Revisit only if native relaxation cannot support the desired
+  model.
+
 ## 2026-06-27 Momentum/Corridor Direction
 
 - The user reframed the target objective: player GPS should optimize for