feat(city): animation primitives and effects

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

crates/city/src/main.rs

@@ -3,6 +3,7 @@
 // Screens, widgets and overlays land task by task; lifted once the app is wired up.
 #![allow(dead_code)]
 
+mod anim;
 mod app;
 mod cli;
 mod save;