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>

xtool.yml

@@ -0,0 +1,13 @@
+# xTool configuration — https://xtool.sh
+# Run `xtool configure` once to set up your Apple ID credentials and Darwin SDK.
+# Then: `xtool run` to build and install directly to a connected device.
+
+targets:
+  SilverApple:
+    bundleIdentifier: uk.silverlabs.silverapple
+    # Fill in your Apple Developer Team ID (10-char string from developer.apple.com)
+    teamID: ""
+    infoPlist: SilverApple/Info.plist
+    entitlements: SilverApple/SilverApple.entitlements
+    deploymentTarget: "16.0"
+    deviceFamily: [iphone]