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>

Cargo.lock

@@ -142,6 +142,7 @@ name = "citygen"
 version = "0.1.0"
 dependencies = [
  "noise",
+ "postcard",
  "rand 0.9.5",
  "rand_chacha",
  "ron",