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>

project.yml

@@ -1,50 +0,0 @@
-name: SilverApple
-
-options:
-  bundleIdPrefix: uk.silverlabs
-  deploymentTarget:
-    iOS: "16.0"
-  xcodeVersion: "15.0"
-  createIntermediateGroups: true
-  groupSortPosition: top
-
-settings:
-  base:
-    SWIFT_VERSION: "5.9"
-    DEVELOPMENT_TEAM: ""  # Fill in your Apple Developer Team ID before building
-    CODE_SIGN_STYLE: Automatic
-    ENABLE_BITCODE: false
-
-targets:
-  SilverApple:
-    type: application
-    platform: iOS
-    deploymentTarget: "16.0"
-    sources:
-      - path: SilverApple
-    settings:
-      base:
-        PRODUCT_BUNDLE_IDENTIFIER: uk.silverlabs.silverapple
-        INFOPLIST_FILE: SilverApple/Info.plist
-        CODE_SIGN_ENTITLEMENTS: SilverApple/SilverApple.entitlements
-        ASSETCATALOG_COMPILER_APPICON_NAME: AppIcon
-        TARGETED_DEVICE_FAMILY: "1"  # iPhone only
-    dependencies:
-      - framework: AuthenticationServices.framework
-        embed: false
-      - framework: SafariServices.framework
-        embed: false
-      - framework: Security.framework
-        embed: false
-
-  SilverAppleTests:
-    type: bundle.unit-test
-    platform: iOS
-    deploymentTarget: "16.0"
-    sources:
-      - path: SilverAppleTests
-    settings:
-      base:
-        PRODUCT_BUNDLE_IDENTIFIER: uk.silverlabs.silverapple.tests
-    dependencies:
-      - target: SilverApple