feat: add spinner component

docs/CHECKLIST.md

@@ -8,13 +8,14 @@ is the first downstream consumer; its screens drive the order.
 
 ## Next
 
-- [ ] **Spinner** (`core`) — the loading ring, indeterminate + determinate.
+- [ ] **Spinner** (`core`) — the loading ring, indeterminate + determinate;
+      painted-ring and rotating-icon variants.
   Replaces `Tab::busy`'s placeholder ring; land before signed wires repo
   loading into dock tabs.
-    - [ ] API sketch
-    - [ ] Implementation (theme-baked, motion inline per AGENTS.md)
-    - [ ] Tests
-    - [ ] Gallery panel
+    - [x] API sketch
+    - [x] Implementation (theme-baked, motion inline per AGENTS.md)
+    - [x] Tests
+    - [x] Gallery panel
     - [ ] Review
 
 ## Queue — to follow