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/city/src/render/mod.rs

@@ -9,7 +9,7 @@ pub mod traffic;
 pub mod viewport;
 pub mod weather;
 
-use citygen::{BuildingKind, ConnectorKind, Pos, StratumKind, Theme};
+use citygen::{BuildingKind, ConnectorKind, StratumKind, Theme};
 use ratatui::buffer::Buffer;
 use ratatui::style::{Color, Modifier, Style};
 use serde::{Deserialize, Serialize};
@@ -95,7 +95,6 @@ pub struct RenderCtx<'a> {
     pub stratum: StratumKind,
     pub light: u8,
     pub tick: u64,
-    pub pos: Pos,
     pub vis: Visibility,
     pub connector: Option<ConnDir>,
     pub kind: Option<BuildingKind>,