Add required array element constraints

doc/manual/souce/design/embedding-api.md

@@ -17,7 +17,7 @@ let / function env
 ```
 
 Module top-level bindings shadow prelude bindings.
-Primitive type names such as `String`, `Int`, `Float`, `Bool`, and `Array` are handled before environment lookup, so they are reserved and cannot be shadowed by host bindings.
+Primitive type names such as `String`, `Int`, `Float`, and `Bool` are handled before environment lookup, so they are reserved and cannot be shadowed by host bindings.
 
 ## Global bindings
 
@@ -59,12 +59,16 @@ HostValue =
   Float
   Bool
   Array(Vec<HostValue>)
+  ArrayConstraint { item, constraints, default }
   Object(Vec<HostField>)
   Abstract { constraints, default }
 ```
 
 When a host value is bound, the engine internalizes it into `RuntimeValue` and allocates value thunks for object fields, array items, and defaults.
 
+`HostValue::array_of(item)` builds an array constraint with a required element schema.
+There is no host API for an unconstrained abstract array.
+
 ## Abstract host objects
 
 A host-provided schema object is represented as a concrete object structure whose fields may contain abstract values.