Add Swift Package Manager support

Add Package.swift with three products: GCDWebServer (core),
GCDWebUploader, and GCDWebDAVServer.

Public headers live across Core/Requests/Responses subdirectories, which
SPM cannot expose directly. To support SPM without moving any files
(keeping the Xcode project and CocoaPods builds intact), public headers
are surfaced through symlinks in include/ directories with a hand-written
umbrella-directory module map; the private GCDWebServerPrivate.h is left
unlinked so it stays out of the public module.

GCDWebUploader.bundle is shipped as a .copy resource and located at
runtime via SWIFTPM_MODULE_BUNDLE under a #if SWIFT_PACKAGE branch.

Verified: swift build (all targets), Swift module imports, Xcode
framework build, and the XCTest suite all pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Package.swift

@@ -0,0 +1,55 @@
+// swift-tools-version:5.9
+import PackageDescription
+
+let package = Package(
+  name: "GCDWebServer",
+  platforms: [
+    .iOS(.v16),
+    .tvOS(.v16),
+    .macOS(.v12),
+  ],
+  products: [
+    .library(name: "GCDWebServer", targets: ["GCDWebServer"]),
+    .library(name: "GCDWebUploader", targets: ["GCDWebUploader"]),
+    .library(name: "GCDWebDAVServer", targets: ["GCDWebDAVServer"]),
+  ],
+  targets: [
+    .target(
+      name: "GCDWebServer",
+      path: "GCDWebServer",
+      // Public headers are exposed via symlinks in include/ so that GCDWebServerPrivate.h stays private.
+      publicHeadersPath: "include",
+      cSettings: [
+        // Source files import sibling headers across these subdirectories with quoted #imports.
+        .headerSearchPath("Core"),
+        .headerSearchPath("Requests"),
+        .headerSearchPath("Responses"),
+      ],
+      linkerSettings: [
+        .linkedLibrary("z"),
+        .linkedFramework("CoreServices", .when(platforms: [.iOS, .tvOS])),
+        .linkedFramework("CFNetwork", .when(platforms: [.iOS, .tvOS])),
+        .linkedFramework("SystemConfiguration", .when(platforms: [.macOS])),
+      ]
+    ),
+    .target(
+      name: "GCDWebUploader",
+      dependencies: ["GCDWebServer"],
+      path: "GCDWebUploader",
+      exclude: [".DS_Store"],
+      resources: [
+        .copy("GCDWebUploader.bundle"),
+      ],
+      publicHeadersPath: "include"
+    ),
+    .target(
+      name: "GCDWebDAVServer",
+      dependencies: ["GCDWebServer"],
+      path: "GCDWebDAVServer",
+      publicHeadersPath: ".",
+      linkerSettings: [
+        .linkedLibrary("xml2"),
+      ]
+    ),
+  ]
+)