refactor(SplitterPanel): tighten expose types to Readonly<Ref<T>>

PHILOSOPHY §4.2 — SplitterPanelExpose.size and .isCollapsed were
typed Ref<T> but implemented as toRef (already readonly). Aligns
the interface with the implementation. No runtime change.

Author: johnleider

packages/0/src/components/Splitter/SplitterPanel.vue

@@ -37,8 +37,8 @@
   export interface SplitterPanelExpose {
     collapse: () => void
     expand: () => void
-    size: Ref<number>
-    isCollapsed: Ref<boolean>
+    size: Readonly<Ref<number>>
+    isCollapsed: Readonly<Ref<boolean>>
   }
 
   export interface SplitterPanelSlotProps {