fix: use composition.reg_id for entity identity in slot_of

get_children() returns fresh userdata wrappers without __eq; raw ==
compares Lua pointers (fails across separate get_children calls).
slot_of now reads composition.reg_id from each child to find the
matching slot, making contains/remove reliable across wrapper instances.

README.md

@@ -97,6 +97,11 @@ Non-item children (future use) are silently excluded.
 
 **Syntax:** `inventory.contains(container: entity, item: entity) -> bool`
 
+**Example:**
+```lua
+local has_rock = inventory.contains(backpack, rock)
+```
+
 **Description:** Returns `true` if `item` is currently a child of
 `container` under a synthetic slot.
 
@@ -104,6 +109,11 @@ Non-item children (future use) are silently excluded.
 
 **Syntax:** `inventory.count(container: entity) -> number`
 
+**Example:**
+```lua
+local n = inventory.count(backpack)
+```
+
 **Description:** Returns the number of items currently in `container`.
 Equivalent to `#inventory.contents(container)`.