feat: add resizable 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.
-- [ ] **Resizable group** — stacked arbitrary panes with drag dividers
-      outside the dock; signed's commit-diff split.
 - [ ] **Searchable list filter** — searchable collection behind combobox and
       list option sources; the combobox's pluggable filter covers part of it
       (signed's `SearchableVec`).
@@ -112,6 +110,8 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
 - [x] Titlebar + window controls — app-owned window dragging, per-platform
       buttons, `Theme::titlebar_height`
 - [x] Sidebar auto-open — `PanelDescriptor::right_sidebar` + `DockSidebar`
+- [x] Resizable group — stacked panes with drag dividers outside the dock,
+      per-pane floors, double-click reset, keyboard steps
 
 ### gallery (`examples/gallery`)