Add advanced Decodal example

examples/advanced/README.md

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+# Advanced Decodal example
+
+This example exercises multiple current Decodal features together:
+
+- multi-file `import`
+- top-level recursive module scope
+- schema/value composition with `&`
+- deep patch with `//`
+- `default` fallback during materialization
+- constrained function parameters
+- lazy function arguments
+- `match` expressions
+- nested dot-path fields
+- arrays
+
+Run it with:
+
+```sh
+cargo run -q -p decodal -- examples/advanced/main.dcdl
+```
+
+The entrypoint is `main.dcdl`.

examples/advanced/main.dcdl

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+let
+  schema = import "./schema.dcdl";
+  profiles = import "./profiles.dcdl";
+
+  mkService = (cfg: schema.Service) =>
+    cfg // {
+      summary = match cfg.env {
+        "prod": "production service";
+        "dev": "development service";
+        _: "custom service";
+      };
+    };
+
+  devService = mkService(
+    profiles.base & {
+      name = "api-dev";
+      port = 8443;
+    }
+  );
+
+  prodService = mkService(
+    profiles.prod & {
+      name = "api";
+      port = 9443;
+    }
+  );
+
+  disabledService = mkService(
+    profiles.disabled & {
+      name = "api-disabled";
+      port = 10443;
+    }
+  );
+in
+{
+  services = [
+    devService,
+    prodService,
+    disabledService,
+  ];
+
+  selected = match "prod" {
+    "prod": prodService;
+    "dev": devService;
+    _: disabledService;
+  };
+}

examples/advanced/profiles.dcdl

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+# Environment-specific patches.
+
+base = {
+  env = "dev";
+  tags = ["decodal", "dev"];
+};
+
+prod = base // {
+  env = "prod";
+  host = "api.internal";
+  feature.limit = 100;
+  tags = ["decodal", "prod"];
+};
+
+disabled = base // {
+  feature.enable = false;
+  tags = ["decodal", "disabled"];
+};

examples/advanced/schema.dcdl

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+# Shared service schema.
+# Top-level fields are recursive, so SecurePort can refer to Port.
+
+Port = Int & >= 1 & <= 65535;
+SecurePort = Port & > 443;
+
+Service = {
+  name = String;
+  env = String default "dev";
+  host = String default "localhost";
+  port = SecurePort default 8443;
+
+  feature = {
+    enable = Bool default true;
+    limit = Int & >= 0 default 10;
+  };
+};