v0.4.5: panel-center audio, preload chat feedback, defensive BE check

- audio: distance attenuation now uses the panel center (width/2, height/2
  offset from the anchor along the renderer's right/up axes) instead of the
  anchor block corner, so a 4x4 panel sounds like it's coming from the
  middle of the screen and not the bottom-left.
- preload: each client now posts a chat line on start / completion / failure
  / cache-hit, so a command-block sequence like /videopreload -> /videoplace
  can be timed against the visible "[videopreload] 완료" message.
- safety: VideoPlayback.tick() verifies the anchor BE still exists at each
  active position and forcibly stops playback if it doesn't — covers any
  edge case where BLOCK_ENTITY_UNLOAD doesn't fire.
- /videopreload feedback now explicitly states "완료 알림 후 재생하세요".

gradle.properties

@@ -5,7 +5,7 @@ org.gradle.configuration-cache=false
 
 # Mod
 mod_id=video_player
-mod_version=0.4.4
+mod_version=0.4.5
 maven_group=com.ejclaw.videoplayer
 archives_base_name=video_player