chore: release core 1.1.13 and plugin_idfa 1.0.3 (SPM fixes)

[MichaelGHSeg]

Releases segment_analytics 1.1.13 and segment_analytics_plugin_idfa 1.0.3, mainly to ship working Swift Package Manager support.

What's in the release

Already on main: #213, #218, #215, #197. This PR adds the rest.

• migrate iOS/macOS to correct SPM structure per Flutter 3.44 docs #213: core's iOS/macOS layout moves to what Flutter 3.44+ expects for SPM.
• chore: widen flutter_fgbg constraint to ">=0.7.1 <0.9.0" for SPM support #218: flutter_fgbg >=0.7.1 <0.9.0. 0.8.0 adds SPM but requires Dart 3.8, so the range keeps Dart 3.7 / Flutter 3.29 working on 0.7.1. Chosen over Bump flutter_fgbg to ^0.8.0 for Swift Package Manager support #216's ^0.8.0 for that reason.
• fix(android): use a unique namespace to avoid collision with analytics-kotlin under AGP 9 #215: Android namespace com.segment.analytics.flutter (AGP 9 collision with analytics-kotlin, Android build fails under AGP 9 when analytics-kotlin is also in the dependency graph: duplicate namespace 'com.segment.analytics' #214).
• fix(web): decode JSON-stringified ajs_anonymous_id from localStorage (#196) #197: web reads ajs_anonymous_id without the JSON quotes analytics.js writes. Affected users' anonymousId changes to match analytics.js; noted in the changelog.
• idfa SPM: same restructure as migrate iOS/macOS to correct SPM structure per Flutter 3.44 docs #213. 1.0.2 shipped the old flat ios/Package.swift + Classes/ layout.
• version.dart: was still 1.1.11, so 1.1.12 reports library.version 1.1.11. Now 1.1.13, with a test that it matches pubspec.yaml.

Tested locally

• Flutter 3.29.2 (Dart 3.7, devbox): core analyze clean, 127 tests pass, resolves flutter_fgbg 0.7.1. idfa analyze clean.
• Flutter 3.47.6 (Dart 3.13), SPM enabled, no CocoaPods installed, throwaway app with path deps on core + idfa:
  • resolves flutter_fgbg 0.8.0
  • Flutter's generated plugin package picks up core, idfa and fgbg as Swift packages from the new layouts
  • flutter build macos succeeds
  • flutter build ios --simulator succeeds; AnalyticsPlugin, PluginIdfaPlugin and FGBGPlugin are linked, with no Podfile or Pods
  • Launched on an iOS 26.4 simulator: core's native getContext returns (os=iOS 26.4.1), and idfa's channel is registered and answering (blocked on the ATT prompt, as expected)
• flutter pub publish --dry-run: 0 warnings for both packages.

Not tested locally: Android (no SDK here), the CocoaPods path. CI doesn't build native code for any platform, so these are the only native checks.

After merge

Publish manually as with 1.1.12: flutter pub publish in packages/core, then packages/plugins/plugin_idfa. publish.yml can't publish to pub.dev yet; that's separate follow-up work.