M2-M8: renderer, playback backends, GUI/network, commands, multi-version build

- M2: VideoAnchorRenderer draws width\u00d7height quad oriented by facing
- M3: VideoBackend interface + JavaCV (reflection) and WaterMedia (probe) backends
- M4: VideoConfigScreen GUI + 4 typed payloads + NBT persistence via ReadView/WriteView
- M5: stick item useOnBlock place/edit, AttackBlockCallback delete, /videoPlace /videoDelete /videoMute
- M6: per-tick distance attenuation gain = volume * clamp(1 - d/16, 0, 1), mute zeroes gain
- M7: WatermediaProbe (reflection-only; reports unavailable until v2 supports 1.21.6+)
- M8: multi-version build script (1.21.6/1.21.7/1.21.8) + Gitea Actions matrix workflow

src/main/resources/fabric.mod.json

@@ -18,7 +18,7 @@
   "depends": {
     "fabricloader": ">=0.16.0",
     "fabric-api": "*",
-    "minecraft": ">=1.21.6",
+    "minecraft": "${target_minecraft}",
     "java": ">=21"
   }
 }