Add typed Decodal derive support

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`, and `Bool` are handled before environment lookup, so they are reserved and cannot be shadowed by host 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.
 
 ## Global bindings
 
@@ -87,6 +87,47 @@ Concrete(Object {
 
 This matches the runtime model used for Decodal source-defined schema objects.
 
+## Typed Rust integration
+
+Hosts can use `decodal-derive` to keep a Rust struct, the Decodal schema, and the decoded result in sync.
+The derive implements two traits from the `decodal` crate:
+
+- `DecodalSchema`: builds a `HostValue` schema that can be passed to `Engine::bind_global`.
+- `DecodalDecode`: decodes materialized `Data` back into the Rust type.
+
+```rust
+use decodal::{DecodalDecode, DecodalSchema, EmptyLoader, Engine};
+use decodal_derive::Decodal;
+
+#[derive(Decodal)]
+struct Service {
+    name: String,
+
+    #[decodal(gt = 443, default = 8443)]
+    port: i64,
+
+    #[decodal(rename = "feature.enable", default = true)]
+    feature_enabled: bool,
+}
+
+let mut engine = Engine::new(EmptyLoader);
+engine.bind_global("Service", Service::decodal_schema())?;
+
+let value = engine.eval_module(module)?;
+let data = engine.materialize(&value)?;
+let service = Service::decodal_decode(&data)?;
+```
+
+Supported field attributes are intentionally small:
+
+- `rename = "path.to.field"`
+- `default`
+- `default = value`
+- numeric constraints: `gt`, `gte`, `lt`, `lte`
+
+The derive does not add host callbacks or reflection.
+It only generates schema construction and typed decoding code.
+
 ## SourceLoader and prelude together
 
 `SourceLoader` and host prelude bindings are independent mechanisms.