Added the ImageWidth enumeration to the Website library target.

Services/Website/Sources/Library/Internal/Enumerations/ImageWidth.swift

@@ -0,0 +1,29 @@
+/// A rendition an image asset is available at, from the narrowest up to the full-size file.
+///
+/// ``StaticFile`` names one file per rendition, and a page's `srcset` offers them all so the browser picks by rendered width.
+/// The declaration order is the `srcset` order: narrowest first.
+enum ImageWidth: CaseIterable {
+    /// The small rendition, 480 pixels wide.
+    case small
+    /// The medium rendition, 800 pixels wide.
+    case medium
+    /// The large rendition: the full-size file, 1200 pixels wide.
+    case large
+}
+
+// MARK: - Extensions
+
+extension ImageWidth {
+
+    // MARK: Computed
+
+    /// The rendition's width in pixels: what the file is resampled to, and what its `srcset` descriptor states.
+    var width: Int {
+        switch self {
+        case .small: 480
+        case .medium: 800
+        case .large: 1200
+        }
+    }
+
+}

Services/Website/Sources/Library/Internal/Types/StaticFile.swift

@@ -14,18 +14,26 @@ struct StaticFile: Asset {
     /// The file's name, without extension.
     let fileName: String
 
+    /// The folder holding the file, or `nil` when it sits in its extensions' own folders.
+    ///
+    /// Imagery sits in a folder per page — `img/index`, `img/about` — rather than in its extension's own.
+    let folder: String?
+
     // MARK: Initializers
 
     /// Declares a static file.
     /// - Parameters:
     ///   - fileName: the file's name, without extension.
+    ///   - folder: the folder holding the file, or `nil` (the default) to use each extension's own folder.
     ///   - fileExtensions: the extensions the file is available with, one file each.
     init(
         _ fileName: String,
+        in folder: String? = nil,
         as fileExtensions: AssetExtension...
     ) {
         self.fileExtensions = fileExtensions
         self.fileName = fileName
+        self.folder = folder
     }
 
 }
@@ -55,6 +63,37 @@ extension StaticFile {
 
 }
 
+// MARK: - Methods
+
+extension StaticFile {
+
+    /// The `srcset` value offering every rendition of a responsive image, narrowest first, ending on the full-size file.
+    ///
+    /// The renditions are named by the given closure, which conventionally gives the `large` one the bare file name and the narrower ones their
+    /// width — as `srcset` files are usually named:
+    ///
+    /// ```swift
+    /// static func portrait(_ width: ImageWidth) -> Self {
+    ///     Self(width == .large ? "portrait" : "portrait-\(width.width)", in: "img/about", as: .jpg, .webp)
+    /// }
+    /// ```
+    /// - Parameters:
+    ///   - fileExtension: the format the renditions are named in.
+    ///   - version: the version token appended to each URL, or `nil` (the default) to leave them unversioned.
+    ///   - rendition: the file naming a given width.
+    /// - Returns: the `srcset` value for that format.
+    static func srcSet(
+        for fileExtension: AssetExtension,
+        version: String? = nil,
+        rendition: (ImageWidth) -> Self
+    ) -> String {
+        ImageWidth.allCases
+            .map { "\(rendition($0).urlPath(for: fileExtension, version: version)) \($0.width)w" }
+            .joined(separator: ", ")
+    }
+
+}
+
 // MARK: - Constants
 
 extension StaticFile {

Services/Website/Tests/Library/Cases/Internal/Enumerations/ImageWidthTests.swift

@@ -0,0 +1,24 @@
+import Testing
+
+@testable import WebsiteLibrary
+
+@Suite(
+    "ImageWidth enumeration",
+    .tags(.enumeration)
+)
+struct ImageWidthTests {
+
+    // MARK: Computed tests
+
+    @Test(arguments: zip(
+        ImageWidth.allCases,
+        [480, 800, 1200]
+    ))
+    func `width`(
+        for imageWidth: ImageWidth,
+        expects width: Int
+    ) {
+        #expect(imageWidth.width == width)
+    }
+
+}

Services/Website/Tests/Library/Cases/Internal/Types/StaticFileTests.swift

@@ -33,6 +33,49 @@ struct StaticFileTests {
         #expect(font.urlPath(for: .woff2) == "/font/a-font-400.woff2")
     }
 
+    /// Imagery sits in a folder per page, so a declared folder replaces the extension's own for every extension the file has.
+    @Test
+    func `declared folder paths`() {
+        let portrait = File("portrait", in: "img/about", as: .jpg, .webp)
+
+        #expect(portrait.relativePath(for: .jpg) == "img/about/portrait.jpg")
+        #expect(portrait.relativePath(for: .webp) == "img/about/portrait.webp")
+    }
+
+    // MARK: Methods tests
+
+    @Test(arguments: zip(
+        [AssetExtension.jpg, .webp],
+        ["jpg", "webp"]
+    ))
+    func `srcset offers every rendition narrowest first`(
+        for fileExtension: AssetExtension,
+        expects suffix: String
+    ) {
+        let srcSet = File.srcSet(for: fileExtension) { width in
+            File(
+                width == .large ? "portrait" : "portrait-\(width.width)",
+                in: "img/about",
+                as: .jpg, .webp
+            )
+        }
+
+        #expect(srcSet == [
+            "/img/about/portrait-480.\(suffix) 480w",
+            "/img/about/portrait-800.\(suffix) 800w",
+            "/img/about/portrait.\(suffix) 1200w"
+        ].joined(separator: ", "))
+    }
+
+    @Test
+    func `srcset versions every rendition when given a version`() {
+        let srcSet = File.srcSet(for: .jpg, version: "0123456789abcdef") { _ in
+            File("portrait", in: "img/about", as: .jpg)
+        }
+
+        #expect(srcSet.components(separatedBy: "?v=0123456789abcdef").count - 1 == ImageWidth.allCases.count)
+    }
+
     // MARK: Constants tests
 
     @Test