feat: Swift Package Manager support (dual SPM + CocoaPods)

Adds SPM support for iOS and macOS alongside the existing CocoaPods
integration, per the Flutter plugin migration layout:

- ios|macos/ffmpeg_kit_flutter_new/Package.swift declare the plugin
  target plus 8 binary targets pointing at prebuilt combined
  XCFramework zips (ios-arm64_arm64e + ios simulator + macos slices)
  on the 8.1.2-full-gpl release, pinned by SHA256 checksum.
- Plugin sources move to Sources/ffmpeg_kit_flutter_new/ with public
  headers under include/ffmpeg_kit_flutter_new/; podspecs updated to
  the new paths (CocoaPods keeps using the existing prepare_command).
- scripts/build_spm_artifacts.sh builds the per-library XCFramework
  zips + checksums.json from a variant's release zips (moves the
  vtool arm64-simulator retagging from user machines into the
  release pipeline).
- example: integration tests that exercise FFmpeg at runtime
  (version query, libx264/GPL encode, ffprobe), iOS deployment
  target raised to the plugin minimum 14.0, stale manual
  ffmpeg_kit_flutter_new.framework reference removed from Runner,
  UIScene/implicit-engine migration by Flutter tooling.

Verified on iOS simulator and macOS, each with SPM enabled and with
CocoaPods (4 configurations, all integration tests passing).

Note: requires the 9 SPM artifacts (8 xcframework zips +
checksums.json) to be uploaded to the 8.1.2-full-gpl GitHub release
before the SPM path resolves for users.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>