Add associative map constraints and range refinement

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

@@ -41,10 +41,10 @@ engine.bind_global(
 A user source can then refer to `Service` without importing it.
 
 ```dcdl
-Service & {
+{
   name = "api";
   port = 9443;
-}
+} as Service
 ```
 
 ## HostValue
@@ -60,6 +60,7 @@ HostValue =
   Bool
   Array(Vec<HostValue>)
   ArrayConstraint { item, constraints, default }
+  MapConstraint { value, constraints, default }
   Object(Vec<HostField>)
   Abstract { constraints, default }
 ```
@@ -69,6 +70,9 @@ When a host value is bound, the engine internalizes it into `RuntimeValue` and a
 `HostValue::array_of(item)` builds an array constraint with a required element schema.
 There is no host API for an unconstrained abstract array.
 
+`HostValue::map_of(value)` builds a map constraint whose arbitrary object field values must satisfy `value`.
+`BTreeMap<String, T>` and, with `std`, `HashMap<String, T>` implement `DecodalSchema`, `DecodalDecode`, and `IntoHostValue` using this representation.
+
 ## Abstract host objects
 
 A host-provided schema object is represented as a concrete object structure whose fields may contain abstract values.