feat: add separator component

docs/CHECKLIST.md

@@ -19,8 +19,6 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
 - [ ] **Editable code field** — syntax-highlighted editable multiline field,
       the code view's highlighter over the input's editing model; signed's
       repo file browser.
-- [ ] **Separator** — standalone horizontal/vertical rule; menu-item
-      separators stay menu-scoped.
 - [ ] **Resizable group** — stacked arbitrary panes with drag dividers
       outside the dock; signed's commit-diff split.
 - [ ] **Searchable list filter** — searchable collection behind combobox and
@@ -81,6 +79,8 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
       platform autohide with a `visibility` override
 - [x] Clipboard button — copies a value, confirms with a brief check mark
       that reverts itself
+- [x] Separator — horizontal and vertical hairline rules; menu separators
+      stay menu-scoped
 
 ### data (`crates/data`)