refactor(md-deck): pos string and yaml copy output

Replace per-axis x1/y1/x2/y2 layer fields with a single
pos string (x1,y1-x2,y2) matching the compact layers format.

When a deck is configured via a yaml/tag codeblock (data-config),
the copy button now emits a yaml/tag block serialized from current
store state so template layers round-trip.

src/doc-entries/md-deck.md

@@ -29,23 +29,19 @@ data-config:
   grid: 5x5
   layers:
     - prop: title
-      x1: 1
-      y1: 1
-      x2: 5
-      y2: 1
+      pos: 1,1-5,1
       font: 12
     - template: |
         **{{name}}** — {{type}}
         {{description}}
-      x1: 1
-      y1: 3
-      x2: 5
-      y2: 8
+      pos: 1,3-5,8
       font: 3
       align: l
 ```
 ````
 
+`pos` 为网格位置 `x1,y1-x2,y2`（1-based，与紧凑 layers 字符串同格式）。
+
 ## 图层格式
 
 `layers` 属性格式为 `字段:起始行,起始列-结束列,字号`，多个图层用空格分隔：