Added language support to the Website service target.

Services/Website/Sources/Library/Internal/Pages/IndexPage.swift

@@ -17,6 +17,9 @@ struct IndexPage {
     /// The locale the page content is localized to.
     let locale: Locale
 
+    /// The public origin the page derives its canonical URL and language alternates from, or `nil` to omit them.
+    let siteOrigin: String?
+
     /// Resolves the page's text from the bundled String Catalog for the page's ``locale``.
     private let localize: Localize
 
@@ -26,15 +29,18 @@ struct IndexPage {
     /// - Parameters:
     ///   - locale: the locale the page content is localized to.
     ///   - assetVersion: the version token appended to the page's asset URLs, or `nil` (the default) to leave them unversioned.
+    ///   - siteOrigin: the public origin the page derives its canonical URL and language alternates from, or `nil` (the default) to omit them.
     ///   - analytics: the analytics tracker embedded in the document head, or `nil` (the default) to omit it.
     init(
         locale: Locale,
         assetVersion: String? = nil,
+        siteOrigin: String? = nil,
         analytics: Analytics? = nil
     ) {
         self.analytics = analytics
         self.assetVersion = assetVersion
         self.locale = locale
+        self.siteOrigin = siteOrigin
         self.localize = .init(bundle: .module)
     }
 
@@ -44,6 +50,36 @@ struct IndexPage {
 
 extension IndexPage: Page {
 
+    // MARK: Constants
+
+    /// The path the page is served at; ``RootController`` registers its route against it, and the page builds its canonical URL from it.
+    static let path = "/"
+
+    // MARK: Computed
+
+    /// The canonical URL of the page's edition in its language, or `nil` when the origin is unknown.
+    ///
+    /// The origin alone for the default language, since canonical URLs carry no trailing slash — `TrailingSlashRedirectMiddleware` enforces
+    /// that on every path but the root, which has nothing to strip.
+    var canonicalURL: String? {
+        siteOrigin.map {
+            Language(of: locale).url(
+                origin: $0,
+                path: Self.path
+            )
+        }
+    }
+
+    /// The site-wide metadata, preceded by the `hreflang` alternates tying the page's language editions together.
+    @HTMLBuilder
+    var metadata: some HTML {
+        languageAlternates(
+            origin: siteOrigin,
+            path: Self.path
+        )
+        siteMetadata
+    }
+
     // MARK: Properties
 
     var content: some HTML {