Structured Data support for the Page protocol in the Infrastructure package (#32)

This PR contains the work done to introduce a `StructuredData` type that pages use to describe themselves to search engines as schema.org JSON-LD, and wires it into the Page protocol so the payload renders automatically in the document head.

Reviewed-on: rock-n-code/loud-amsterdam#32
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>

Packages/Infrastructure/Sources/Public/Protocols/Page.swift

@@ -4,8 +4,8 @@ import Foundation
 /// A page of a website: an HTML document with the shared scaffolding assembled around the page's content.
 ///
 /// A conforming page supplies its locale, its title, the stylesheets and scripts it needs, its head metadata, and its content; the protocol assembles the
-/// rest of the document around them: the viewport declaration, the summary, canonical, and social card tags, and the metadata followed by the
-/// stylesheet links in the head, and the content followed by the script tags in the body.
+/// rest of the document around them: the viewport declaration, the summary, canonical, and social card tags, the structured data script, and the
+/// metadata followed by the stylesheet links in the head, and the content followed by the script tags in the body.
 public protocol Page: HTMLDocument, Sendable {
 
     // MARK: Associated types
@@ -42,6 +42,9 @@ public protocol Page: HTMLDocument, Sendable {
     /// to omit them.
     var socialCard: SocialCard? { get }
 
+    /// The page's structured data, rendered as a JSON-LD script in the document head, or `nil` (the default) to omit it.
+    var structuredData: StructuredData? { get }
+
     /// The stylesheets linked in the document head, in order.
     var stylesheets: [any Asset] { get }
 
@@ -74,11 +77,14 @@ public extension Page {
         }
     }
     
-    /// The viewport declaration, the ``summary``, ``canonicalURL``, and ``socialCard`` tags (when provided), and the ``metadata``
-    /// followed by the ``stylesheets`` links, placed in the document head.
+    /// The viewport declaration, the ``summary``, ``canonicalURL``, and ``socialCard`` tags and the ``structuredData`` script
+    /// (when provided), and the ``metadata`` followed by the ``stylesheets`` links, placed in the document head.
     ///
     /// The charset declaration is omitted: Elementary's `HTMLDocument` scaffolding already emits `<meta charset="UTF-8">` before this markup,
     /// and HTML5 allows only one.
+    ///
+    /// The structured data is an inert data block — browsers never execute it, so a site's `Content-Security-Policy` does not apply to it —
+    /// that search engines read for the organization's name, logo, and profiles.
     @HTMLBuilder
     var head: some HTML {
         meta(
@@ -112,6 +118,15 @@ public extension Page {
             }
         }
 
+        if let structuredData {
+            script(.custom(
+                name: "type",
+                value: "application/ld+json"
+            )) {
+                HTMLRaw(structuredData.payload)
+            }
+        }
+
         metadata
         
         for file in stylesheets {
@@ -130,6 +145,11 @@ public extension Page {
         nil
     }
 
+    /// The structured data is omitted unless the page provides one.
+    var structuredData: StructuredData? {
+        nil
+    }
+
     /// The summary is omitted unless the page provides one.
     var summary: String? {
         nil