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.

build.gradle

@@ -7,8 +7,25 @@ plugins {
 version = project.mod_version
 group = project.maven_group
 
+// Optional platform bundle target: `-Pplatform=windows-x86_64` etc. produces a fat jar
+// with that platform's JavaCV + FFmpeg natives nested inside via Fabric's `include`
+// directive. Without `-Pplatform`, the build matches pre-0.4.8 behavior (small jar,
+// user supplies JavaCV separately).
+def javacvPlatform = (project.findProperty('platform') ?: '').toString()
+def bundleJavaCv = !javacvPlatform.isEmpty()
+
+def javacvVersion = '1.5.13'
+def ffmpegVersion = '8.0.1-1.5.13'
+def supportedPlatforms = ['windows-x86_64', 'linux-x86_64', 'macosx-x86_64', 'macosx-arm64']
+if (bundleJavaCv && !supportedPlatforms.contains(javacvPlatform)) {
+    throw new GradleException(
+        "Unknown -Pplatform=${javacvPlatform}. Allowed: ${supportedPlatforms.join(', ')}")
+}
+
 base {
-    archivesName = project.archives_base_name
+    archivesName = bundleJavaCv
+        ? "${project.archives_base_name}-${javacvPlatform}"
+        : project.archives_base_name
 }
 
 repositories {
@@ -26,6 +43,19 @@ dependencies {
 
     implementation "net.fabricmc:fabric-loader:${project.loader_version}"
     implementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
+
+    if (bundleJavaCv) {
+        // Nest the same 5 jars that the README's manual install step used to require, so
+        // users no longer need -Xbootclasspath/a. `include` adds them as jar-in-jar entries;
+        // Fabric loader unpacks and classloads them at runtime.
+        // `transitive = false` keeps us from dragging in opencv/openblas/etc — we only use
+        // FFmpegFrameGrabber / Frame / Java2DFrameConverter.
+        include(implementation("org.bytedeco:javacv:${javacvVersion}") { transitive = false })
+        include(implementation("org.bytedeco:javacpp:${javacvVersion}") { transitive = false })
+        include(implementation("org.bytedeco:ffmpeg:${ffmpegVersion}") { transitive = false })
+        include(implementation("org.bytedeco:javacpp:${javacvVersion}:${javacvPlatform}") { transitive = false })
+        include(implementation("org.bytedeco:ffmpeg:${ffmpegVersion}:${javacvPlatform}") { transitive = false })
+    }
 }
 
 processResources {