fix: review findings

- screen transitions snapshot the previous frame (ratatui swaps buffers
  after draw, so the old snapshot was always blank); frame loop extracted
  into a testable Runner
- .city loading rejects structurally inconsistent files: Grid deserialize
  checks size vs data, load_city checks header vs body and runs
  City::validate (now also checks Net link endpoints)
- saving from the map hands the live map screen back instead of a fresh
  explore screen
- signage glyphs covered by the unicode-width test

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

crates/citygen/src/validate.rs

@@ -152,6 +152,14 @@ impl City {
             }
         }
 
+        for link in &self.net.links {
+            for id in [link.0, link.1] {
+                if id.index() >= self.net.nodes.len() {
+                    out.push(Violation::NetAnchor(id));
+                }
+            }
+        }
+
         for d in &self.districts {
             let has_building = self.buildings.iter().any(|b| b.district == d.id);
             let has_public = self.connectors.iter().any(|c| {