docs: README, dead-code cleanup, smoke fixes

- README with run instructions and key reference
- lift the crate-level dead_code allowance; scope it to the anim library surface
- render paths tolerate a city that is still generating (district/building ids
  without entries), found by driving the app through a pty
- status messages show in the status bar, not only inside the inspector
- buffered JSON export (26 s -> 0.4 s for a large city)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/citygen/src/world.rs

@@ -366,13 +366,23 @@ impl City {
         self.connectors.iter().filter(move |c| c.pos == p)
     }
 
+    /// Panics if the id is unknown; use `district_opt` on a city that may still be generating.
     pub fn district(&self, id: DistrictId) -> &District {
         &self.districts[id.index()]
     }
 
+    pub fn district_opt(&self, id: DistrictId) -> Option<&District> {
+        self.districts.get(id.index())
+    }
+
+    /// Panics if the id is unknown; use `building_opt` on a city that may still be generating.
     pub fn building(&self, id: BuildingId) -> &Building {
         &self.buildings[id.index()]
     }
+
+    pub fn building_opt(&self, id: BuildingId) -> Option<&Building> {
+        self.buildings.get(id.index())
+    }
 }
 
 #[cfg(test)]