Antiz96/oniri ? reverse-engineered prompt

Build me a small Rust app for niri that automatically keeps the only window in a workspace maximized. I want it to run in the background from my niri startup config, and it should watch window changes so when I open the first app it gets maximized, and when I close everything back down the last remaining window gets maximized again.

Please also support a mode where only the first window gets maximized, and another mode that feels more like a tiling layout, where the first window stops being maximized when a second one opens. It should be easy to start from the command line, have a clear help message, and include the right config examples for niri.

If you need to check the current niri IPC docs or behavior online, go ahead. Also include a man page and shell completion files if that fits naturally.