FFmpeg software H.264 decoder: opt-in via pubspec flag

- Gate buildFfmpegJni + jniLibs packaging on `ux: enable_ffmpeg` in the
  consuming app's pubspec (default off) -- no LGPL / H.264-patent
  exposure unless explicitly enabled
- appInfoBuilder generates kUxEnableFfmpeg from the same flag so apps
  register the FFmpeg LGPL notice eagerly, pubspec-only (no dart-define)
- Add registerFfmpegLicense() + bundled LGPL-2.1 text asset
- FFmpeg compliance docs (LICENSES-3RDPARTY.md, android/ffmpeg/README.md)
- Network video streaming: XVideoPlayerController.network

pubspec.yaml

@@ -20,7 +20,7 @@ dependencies:
   flutter:
     sdk: flutter
   matcher: ^0.12.16
-  build: ^2.4.0
+  build: '>=2.4.0 <5.0.0'
   clock: ^1.1.1
   ffi: ^2.1.0
 
@@ -39,3 +39,7 @@ flutter:
         pluginClass: XPlugin
       macos:
         pluginClass: XPlugin
+  assets:
+    # LGPL-2.1 text for the bundled FFmpeg software H.264 decoder
+    # (libffmpegJNI.so, Android only). Surfaced via registerFfmpegLicense().
+    - assets/licenses/ffmpeg_LGPL-2.1.txt