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.

doc/manual/souce/design/runtime-model.md

@@ -123,12 +123,16 @@ constraint は concrete value とは別の型として扱う。
 ```text
 Constraint =
   Type(PrimitiveType)
+  ArrayItems(ThunkId)
   Compare(Op, Literal)
   Regex(Pattern)
   BuiltinPredicate(Symbol)
   ObjectConstraint(...)
 ```
 
+`ArrayItems` は配列そのものの型と、すべての要素へ合成する schema thunk を表す。
+配列用の primitive type は持たず、抽象配列には必ず要素制約が必要である。
+
 初期実装では、object の形は主に `Concrete(Object)` の field に `Abstract` を置くことで表現する。
 object 全体にかかる constraint は必要になった時点で追加する。