feat(citygen): City::validate and seed sweep tests

Also guarantees one building per district and repairs interiors split
by crossing partitions, both found by the new validation.

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

crates/citygen/Cargo.toml

@@ -11,3 +11,6 @@ rand_chacha.workspace = true
 noise.workspace = true
 thiserror.workspace = true
 ron.workspace = true
+
+[dev-dependencies]
+postcard.workspace = true