Function primary_modifier 

pub fn primary_modifier(mods: &ModifiersState) -> bool

Returns true when only the platform’s primary modifier key is held (no Shift, no Alt, and no cross modifier), i.e.:

• macOS: Cmd pressed; Shift, Alt, and Ctrl not pressed
• Windows/Linux: Ctrl pressed; Shift, Alt, and Super not pressed

Use this for single-key shortcuts (Cmd+T on macOS / Ctrl+T elsewhere). Excluding the cross modifier prevents e.g. Ctrl+Cmd+T from triggering a hardcoded Cmd+T handler on macOS, which would otherwise shadow any registered Ctrl+Cmd+T keybinding.