immutable properties complete and tested

src/database/object.rs

@@ -149,6 +149,9 @@ pub struct SchemaObject {
   #[serde(default)]
   #[serde(skip_serializing_if = "Option::is_none")]
   pub extensible: Option<bool>,
+  #[serde(default)]
+  #[serde(skip_serializing_if = "Option::is_none")]
+  pub immutable: Option<bool>,
 
   // Contains ALL structural fields perfectly flattened from the ENTIRE Database inheritance tree (e.g. `entity` fields like `id`) as well as local fields hidden inside conditional `cases` blocks.
   // This JSON exported array gives clients absolute deterministic visibility to O(1) validation and masking bounds without duplicating structural memory.
@@ -306,4 +309,8 @@ impl SchemaObject {
 
     false
   }
+
+  pub fn is_immutable(&self) -> bool {
+    self.immutable == Some(true)
+  }
 }