macOS RivetMenuBarController: no separators, items fixed at install, title-only (no icon/click action)

[turinglambdaai]

Dogfooding notes from two app rewrites (brainfuel, movebit):

1. install(title:menuItems:) takes a flat [(String, String, () -> Void)] — no separator support, so tray menus lose their grouping (old apps had \─\ separators).
2. Items are fixed at install time: movebit needs to swap 暂停提醒 1 小时 ↔ 恢复提醒 (and localized labels) on pause-state change, so it re-installs the whole controller. A small update/replace API (by key) or a menu-builder closure would avoid the churn.
3. Title-only: parity apps want an icon (template NSImage) in the status bar plus a tooltip; and the old tray behavior maps left-click to "show the window", which a status item's menu-at-click doesn't express.

Suggestion: evolve RivetMenuBarController with: separators (a reserved key or an enum item type), update(items:) / setItem(_:title:), optional button.image, optional button.action for click-without-menu, and button.toolTip. Also worth mirroring on the backend side: set-tray-menu! currently can't express separators either.