Added an optional per-asset folder to the Asset protocol and the AssetExtension extension in the Infrastructure package.

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

@@ -1,11 +1,15 @@
 /// An asset shipped with a website: a file stored under the static files root and served by Hummingbird's `FileMiddleware` middleware.
 ///
 /// A conforming asset supplies its file name and the extensions it is available with, each resolving to its own file; the protocol derives the paths from them:
-/// the file's path within the static files root and the URL path it is served at, optionally versioned to bust caches.
+/// the file's path within the static files root and the URL path it is served at, optionally versioned to bust caches. Each file lands in its extension's own
+/// folder unless the asset names a ``folder`` of its own.
 public protocol Asset: Sendable {
 
     // MARK: Properties
 
+    /// The folder within the static files root that holds the asset's files, or `nil` (the default) to use each extension's own folder.
+    var folder: String? { get }
+
     /// The file extensions the asset is available with.
     var fileExtensions: [AssetExtension] { get }
 
@@ -18,6 +22,11 @@ public protocol Asset: Sendable {
 
 public extension Asset {
 
+    // MARK: Computed
+
+    /// The asset's files live in each extension's own folder by default.
+    var folder: String? { nil }
+
     // MARK: Methods
 
     /// Resolves the asset's path against the given base directory.
@@ -50,7 +59,7 @@ public extension Asset {
     ) -> String {
         let file = "\(fileName).\(fileExtension.rawValue)"
 
-        return fileExtension.subdirectory
+        return (folder ?? fileExtension.folder)
             .map { "\($0)/\(file)" } ?? file
     }