chore(build): migrate to SwiftPM + xTool, add AltStore/SideStore pipeline

- Replace XcodeGen project.yml with Package.swift (SwiftPM)
- Add xtool.yml for xTool cross-platform build configuration
- Add .gitea/workflows/release.yml — builds IPA on tagged release via
  self-hosted macOS runner, uploads to Gitea releases, auto-updates source
- Add altstore-source.json — dual-compatible with AltStore and SideStore
  (top-level downloadURL, ISO 8601 dates, SilverLABS branding)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Package.swift

@@ -0,0 +1,32 @@
+// swift-tools-version: 5.9
+import PackageDescription
+
+let package = Package(
+    name: "SilverApple",
+    platforms: [
+        .iOS(.v16)
+    ],
+    targets: [
+        .executableTarget(
+            name: "SilverApple",
+            path: "SilverApple",
+            exclude: [
+                "Info.plist",
+                "SilverApple.entitlements",
+            ],
+            resources: [
+                .process("Assets.xcassets"),
+            ],
+            linkerSettings: [
+                .linkedFramework("AuthenticationServices"),
+                .linkedFramework("SafariServices"),
+                .linkedFramework("Security"),
+            ]
+        ),
+        .testTarget(
+            name: "SilverAppleTests",
+            dependencies: ["SilverApple"],
+            path: "SilverAppleTests"
+        ),
+    ]
+)