Refactor AttributeGraphVendor from enum to RawRepresentable struct

Change AttributeGraphVendor from a String-backed enum to a struct
conforming to RawRepresentable and CaseIterable, aligning with the
pattern used by RenderBoxVendor. Add DocC comments.

Author: Kyle-Ye

Sources/OpenAttributeGraphShims/GraphShims.swift

@@ -2,10 +2,26 @@
 //  GraphShims.swift
 //  OpenAttributeGraphShims
 
-public enum AttributeGraphVendor: String {
-    case oag = "org.OpenSwiftUIProject.OpenAttributeGraph"
-    case ag = "com.apple.AttributeGraph"
-    case compute = "dev.incrematic.compute"
+/// A type that identifies the underlying attribute graph implementation vendor.
+///
+/// Use `attributeGraphVendor` to check which vendor is active at runtime.
+public struct AttributeGraphVendor: RawRepresentable, CaseIterable {
+    public let rawValue: String
+
+    public init(rawValue: String) {
+        self.rawValue = rawValue
+    }
+
+    /// OpenAttributeGraph — the open-source implementation.
+    public static let oag = AttributeGraphVendor(rawValue: "org.OpenSwiftUIProject.OpenAttributeGraph")
+
+    /// Apple's private AttributeGraph framework.
+    public static let ag = AttributeGraphVendor(rawValue: "com.apple.AttributeGraph")
+
+    /// Incrematic Compute engine.
+    public static let compute = AttributeGraphVendor(rawValue: "dev.incrematic.compute")
+
+    public static var allCases: [AttributeGraphVendor] { [.oag, .ag, .compute] }
 }
 
 #if OPENATTRIBUTEGRAPH_COMPUTE