Make helpers public and improve Nix packaging

Export `block_title` and `helpers` with `pub mod` for external use.
Add Nix improvements: version check hook, update script, install check.
Include post‑install step to copy soundtracks and rename binary.
Extend meta with platforms, mainProgram, and maintainer handling.
Update docs to show required imports for block title helpers.

src/app/mod.rs

@@ -1,6 +1,6 @@
 mod app;
 mod buildings;
-mod helpers;
+pub mod helpers;
 mod keybind;
 mod keybindings;
 mod state;