feat(simulation): add Layer-2 inter-settlement road/rail graph (T-1038)

New deterministic cascade layer (CascadeLayer::RoadGraph) building the
transport topology between Layer-3 settlement placements, stored on
BodyWorldState.road_graph (session-cached, never serialized).

- MST force-connect (strict Kruskal over squared distance) + secondary
  links across a 30-60% body-scale band where the MST detours >1.5x.
- A* over the Layer-1 terrain-cost field on a coarse ~64-wide routing grid:
  ocean/lake impassable, river corridors 1/2 cost, slope >=35deg (D-210
  roughness ~0.7) 10x, else the D-210 terrain_modification_cost. Columns
  wrap (equirectangular); integer costs + index tie-break keep A*
  deterministic.
- MaintenanceAuthority per edge (new generator.rs enum) from endpoint
  political archetypes + haul length; waypoint nodes at long-edge midpoints;
  named-route identity join (designed-for, pool empty post-D-223); junction
  detection (high_connectivity_junctions, degree>=3).

Wired into the cascade (gen_queue requests CascadeLayer::RoadGraph). Adds
Copy to PoliticalArchetype. 10 road_graph unit tests + a cascade e2e test;
full suite + clippy --all-targets -D warnings green.

Deferred to T-1076 (gated on the D-242 settlement model): the RailHeadFacing
junction pass (needs a D-213 FoundingOrientation amendment + Layer-4 handling)
and the hub-selection refinement (needs baked population/specialization).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/gen_queue.rs

@@ -374,15 +374,13 @@ fn run_work_item(item: &GenWorkItem) -> GenCompletion {
                 } else {
                     hm
                 };
-                // Run through RegionProfile (T-1023, D-239 §1): includes Settlement
-                // and all prior layers. RegionProfile > Settlement in CascadeLayer ord
-                // so Settlement also runs when body_params is Some. When body_params
-                // is None the cascade falls back to Settlement as the terminal layer.
-                let up_to = if body_params.is_some() {
-                    CascadeLayer::RegionProfile
-                } else {
-                    CascadeLayer::Settlement
-                };
+                // Run the full cascade through RoadGraph (Layer 2, T-1038), the
+                // terminal layer. It subsumes Settlement, RegionProfile (T-1023),
+                // and all prior layers. RegionProfile derivation still gates on
+                // body_params internally (skipped when absent — e.g. a body with no
+                // params row), but the road graph needs no body params, so it runs
+                // for every analyzed body.
+                let up_to = CascadeLayer::RoadGraph;
                 let snapshot = run_cascade_from_heightmap(
                     *body_seed,
                     working,