feat(simulation): run Layer-3 settlement placement in the runtime cascade (#955)

Wire the existing D-211 attractor-matching engine into the live
generation cascade so settlements are placed in-game, not just in tests.

- CityContextReader::read_body_settlements reads a body's settlements
  from atlas_city_names (ordered by id for determinism). NULL
  settlement_class defaults to PopulationBudget, not NameLocked: the
  class is NULL until placement runs, and NameLocked would force every
  settlement Tier-A in match_cities and collapse population tiering
  (D-211). NULL economic_role falls back to residential.
- The AnalyzeBody work item carries the body's Vec<CityRecord>, and
  run_work_item now runs up_to Settlement (was Topography). A body with
  no settlements yields empty placements at negligible cost.
- The atlas layer proxy reads settlements on a cache miss and pins them
  onto the work item, keeping the Rayon task DB-free (D-225). A read
  failure is non-fatal: log and place no cities (Layer 1 still runs).
  Threaded through a new CityContextReaderResource Bevy resource opened
  in main.rs, mirroring BodySourceResolverResource.

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

server/src/main.rs

@@ -198,6 +198,23 @@ fn main() {
         ),
     }
 
+    // Settlement reader for Layer-3 placement (#955): reads a body's settlements
+    // from systems.db on a cache miss so the cascade work item stays DB-free.
+    match settled_reach_server::atlas::city_context_reader::CityContextReader::open(
+        &systems_db_path,
+    ) {
+        Ok(reader) => {
+            tracing::info!("City context reader opened: {:?}", systems_db_path);
+            app.insert_resource(
+                settled_reach_server::atlas::city_context_reader::CityContextReaderResource(reader),
+            );
+        }
+        Err(e) => tracing::warn!(
+            "City context reader unavailable ({}). Settlements will not be placed.",
+            e
+        ),
+    }
+
     // Initialize SQLite settings store (#627).
     // Path: alongside save files in the server's working directory.
     let settings_path = std::path::PathBuf::from("settings.db");