update doc

data-structure-database.md

@@ -32,6 +32,7 @@ Every item in the `items` array shares this base shape:
 
 ```json
 {
+  "id": 0,
   "position": { "x": 0.0, "y": 0.0, "z": 0.0 },
   "rotation": { "x": 0.0, "y": 0.0, "z": 0.0 },
   "scale":    { "x": 1.0, "y": 1.0, "z": 1.0 },
@@ -44,6 +45,8 @@ Every item in the `items` array shares this base shape:
 
 > **Coordinate system note:** The X-axis is flipped on save (`x` = `-creator_x`) and un-flipped on load. Y-axis rotation is also negated.
 
+> **`id`:** Stable numeric identifier assigned when the item is created in the Creator (the key of the in-memory `SceneItems` record). It is preserved across save/load rather than derived from array position, so group membership and block-coding references (`sourceItemId`/`targetItemId`) keep pointing at the right item even after items are added or removed. Rooms saved before this field existed fall back to array index on load.
+
 ### `ItemArg`
 
 ```json
@@ -122,6 +125,7 @@ All values are serialized as strings.
 
 ```json
 {
+  "id": 0,
   "position": { "x": 0, "y": 5, "z": 0 },
   "rotation": { "x": 0, "y": 0, "z": 0 },
   "scale":    { "x": 1, "y": 1, "z": 1 },
@@ -140,6 +144,7 @@ All values are serialized as strings.
 
 ```json
 {
+  "id": 1,
   "position": { "x": 0, "y": 4, "z": 0 },
   "rotation": { "x": 0, "y": 0, "z": 0 },
   "scale":    { "x": 1, "y": 1, "z": 1 },
@@ -157,6 +162,7 @@ All values are serialized as strings.
 
 ```json
 {
+  "id": 2,
   "position": { "x": -4.0, "y": 1.0, "z": 0.0 },
   "rotation": { "x": 0.0, "y": 0.0, "z": 0.0 },
   "scale":    { "x": 1.0, "y": 1.0, "z": 1.0 },