feat(citygen): params validation, GenError, seeded rng helpers

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

crates/citygen/src/validate.rs

@@ -0,0 +1,14 @@
+//! City invariants.
+
+use crate::world::City;
+
+/// A broken invariant found by [`City::validate`].
+#[derive(Clone, Debug, PartialEq)]
+pub struct Violation(pub String);
+
+impl City {
+    /// Checks the invariants every finished city must satisfy.
+    pub fn validate(&self) -> Result<(), Vec<Violation>> {
+        Ok(())
+    }
+}