fix(layer-editor): use stable _key for drag-and-drop sorting

Add a unique `_key` to each layer config to maintain stable sortable IDs
across reorder operations. Update LayerRow to use the sortableId prop
and the `use:sortable` directive. Refactor layer-crud utilities to
generate and propagate `_key` values on creation and initialization.

content/deck-test.md

@@ -2,6 +2,8 @@
 
 :md-deck[./names.csv]{size="54x86" grid="5x8" bleed="1" padding="2" layers="name1:1,1-2,2f12 name2:4,7-5,8f12s num1:1,3-2,4f12 num2:4,5-5,6f12s"}
 
+:md-deck[./names.csv]{size="54x86" grid="5x8" layers="num1:1,1-2,2 num2:4,5-5,6f12s name1:1,1-2,2 name2:1,1-2,2 name1:1,1-2,2" }
+
 ## 使用 yaml/tag 语法创建 md-deck
 
 ```yaml/tag
@@ -12,4 +14,4 @@ grid: 5x8
 bleed: 1
 padding: 2
 layers: name1:1,1-2,2f12 name2:4,7-5,8f12s num1:1,3-2,4f12 num2:4,5-5,6f12s
-```
+```