Add Metadata: full App Router metadata binding

src/Metadata.res

@@ -0,0 +1,76 @@
+/**
+ * Typed subset of the Next.js App Router Metadata API — the `metadata` export
+ * and the `generateMetadata` return value — following the official
+ * recommendations: the TITLE TEMPLATE lives in the root layout and pages
+ * return plain title strings (or opt out with an absolute title);
+ * `metadataBase` lives in the root layout so per-page canonical and OpenGraph
+ * URLs can stay relative. Structural: Next consumes the plain object shape.
+ *
+ * Supersedes the minimal `Next.Metadata` (kept for compatibility).
+ *
+ * @example
+ *
+ *  // Root layout
+ *  let metadata: Metadata.t = {
+ *    metadataBase: Metadata.makeUrl("https://example.com"),
+ *    title: Metadata.titleTemplate({default: "Site", template: "%s | Site"}),
+ *    description: "Default description",
+ *    openGraph: {siteName: "Site", type_: "website"},
+ *    icons: [{rel: "icon", url: "/favicon.ico", sizes: "32x32"}],
+ *  }
+ *
+ *  // Page-level generateMetadata
+ *  let generateMetadata = async (props): Metadata.t => {
+ *    ...
+ *    {
+ *      title: Metadata.title(pageTitle), // templated by the root layout
+ *      alternates: {canonical: path},    // relative, resolved by metadataBase
+ *    }
+ *  }
+ */
+// metadataBase must be a JS URL instance.
+type url
+@new external makeUrl: string => url = "URL"
+
+// `title` is string | {default, template} | {absolute}: three runtime shapes
+// behind one abstract type, with typed constructors.
+type title
+type titleTemplate = {default: string, template: string}
+type titleAbsolute = {absolute: string}
+external title: string => title = "%identity"
+external titleTemplate: titleTemplate => title = "%identity"
+external titleAbsolute: titleAbsolute => title = "%identity"
+
+type alternates = {canonical?: string}
+
+type ogImage = {url: string, width?: int, height?: int, alt?: string}
+
+type openGraph = {
+  title?: string,
+  description?: string,
+  url?: string,
+  siteName?: string,
+  locale?: string,
+  @as("type") type_?: string,
+  images?: array<ogImage>,
+}
+
+type icon = {
+  rel: string,
+  url: string,
+  @as("type") type_?: string,
+  sizes?: string,
+}
+
+type robots = {index?: bool, follow?: bool}
+
+type t = {
+  metadataBase?: url,
+  title?: title,
+  description?: string,
+  alternates?: alternates,
+  openGraph?: openGraph,
+  icons?: array<icon>,
+  manifest?: string,
+  robots?: robots,
+}

src/Next.res

@@ -339,7 +339,12 @@ module Headers = {
 }
 
 /**
- * Module for handling Next.js metadata configuration
+ * Module for handling Next.js metadata configuration.
+ *
+ * DEPRECATED: kept only to support older code. New code should use the
+ * top-level `Metadata` module instead, which covers the full App Router
+ * Metadata API (metadataBase, title templates, canonical, OpenGraph, robots)
+ * and is a superset of this one.
  */
 module Metadata = {
   /**