Add reusable segmented inbox filters

docs/DESIGN_SYSTEM.md

@@ -735,3 +735,19 @@ language such as `UPLOAD NEW AVATAR`, `REVISE CONTRIBUTION`, or
 `VIEW BUILD & REPLY`; never substitute a generic destination label when the
 notification type is known. The same response label appears in the detail
 modal. On mobile, response actions become full-width 44-pixel targets.
+
+Inbox filtering uses additive segmented selectors derived from the OS Protocol
+quality control. Channels and message states are independent checkbox-backed
+segments, all selected by default, and each selection change applies
+immediately without a submit button. Selected segments use the orange lower
+rail and filled square indicator; deselected segments remain readable and
+available. The channel rail collapses from six to three and then two columns as
+space narrows, with minimum 44-pixel mobile targets.
+
+Use the shared `SegmentedChoice` component for compact, finite option sets.
+Radio mode represents one required choice; checkbox mode represents additive
+independent choices. Prefer this visible segmented treatment over a dropdown
+when the complete option set is small enough to scan comfortably. Dropdowns
+remain appropriate for long, dynamic, or search-oriented lists. Both modes
+share hover, pressed, focus-visible, selected, disabled, responsive, and
+44-pixel mobile-target behavior so feature modules do not reimplement it.