v0.4.17: music_quiz datapack handshake (server + per-player presence)

Implements docs/mc_video_player_mod_integration.md.

Server (MusicQuizPresence):
- every server tick: #server mq_video_mod = 1
- on MqHelloPayload receive: <player> mq_video_mod = 1
- both silently skip if objective absent (datapack not yet loaded)

Client (MusicQuizClient):
- send MqHelloPayload(1) on JOIN
- resend every 100 client ticks (~5 s) — mandatory because the datapack's
  mq:players/login resets per-player score to 0 at spawn-dialog passage

Payload: video_player:mq_hello, single VAR_INT body (version=1).

gradle.properties

@@ -5,7 +5,7 @@ org.gradle.configuration-cache=false
 
 # Mod
 mod_id=video_player
-mod_version=0.4.16
+mod_version=0.4.17
 maven_group=com.ejclaw.videoplayer
 archives_base_name=video_player