v0.4.8: bundle JavaCV per-platform via Fabric jarJar (no separate install)

- build.gradle: optional -Pplatform=<id> property switches the build into
  a fat-jar mode where javacv 1.5.13 + javacpp + ffmpeg 8.0.1 (java + the
  picked platform's native jar) are all nested into the mod jar via
  Fabric loom's `include` directive. Fabric loader unpacks them at
  runtime, so users no longer need -Xbootclasspath/a:... or 5 separate
  jars in .minecraft/libraries.
- Without -Pplatform, the build produces the same small ~85KB vanilla
  jar as before, so devs/server-side and bring-your-own-JavaCV setups
  still work.
- Per-platform artifacts: video_player-<platform>-0.4.8.jar where
  <platform> ∈ windows-x86_64 / linux-x86_64 / macosx-x86_64 /
  macosx-arm64. Sizes 21-32MB.
- README: STEP 5 (the long JavaCV manual-install + -Xbootclasspath
  section) is gone. New STEP 4 just says 'pick the jar for your OS'.
  Also added a warning about removing the old -Xbootclasspath JVM arg
  when upgrading, since duplicate JavaCV on the boot classpath can
  silently break decoding.

gradle.properties

@@ -5,7 +5,7 @@ org.gradle.configuration-cache=false
 
 # Mod
 mod_id=video_player
-mod_version=0.4.7
+mod_version=0.4.8
 maven_group=com.ejclaw.videoplayer
 archives_base_name=video_player