feat: render GeckoLib cosmetics on player bodies

src/main/java/de/fullrisk/citrusDev/client/cosmetic/PlayerCosmeticModel.java

@@ -0,0 +1,29 @@
+package de.fullrisk.citrusDev.client.cosmetic;
+
+import com.geckolib.model.GeoModel;
+import com.geckolib.renderer.base.GeoRenderState;
+import net.minecraft.resources.Identifier;
+
+public final class PlayerCosmeticModel extends GeoModel<PlayerCosmetic> {
+    private static final Identifier MODEL = Identifier.fromNamespaceAndPath(
+            "citrus-dev", "geckolib/models/player/backpack.geo.json");
+    private static final Identifier TEXTURE = Identifier.fromNamespaceAndPath(
+            "citrus-dev", "textures/cosmetic/backpack.png");
+    private static final Identifier ANIMATION = Identifier.fromNamespaceAndPath(
+            "citrus-dev", "geckolib/animations/backpack.animation.json");
+
+    @Override
+    public Identifier getModelResource(GeoRenderState renderState) {
+        return MODEL;
+    }
+
+    @Override
+    public Identifier getTextureResource(GeoRenderState renderState) {
+        return TEXTURE;
+    }
+
+    @Override
+    public Identifier getAnimationResource(PlayerCosmetic animatable) {
+        return ANIMATION;
+    }
+}

src/main/java/de/fullrisk/citrusDev/client/cosmetic/PlayerCosmeticRenderLayer.java

@@ -0,0 +1,57 @@
+package de.fullrisk.citrusDev.client.cosmetic;
+
+import com.mojang.blaze3d.vertex.PoseStack;
+import net.minecraft.client.model.player.PlayerModel;
+import net.minecraft.client.renderer.SubmitNodeCollector;
+import net.minecraft.client.renderer.entity.RenderLayerParent;
+import net.minecraft.client.renderer.entity.layers.RenderLayer;
+import net.minecraft.client.renderer.entity.state.AvatarRenderState;
+import net.minecraft.client.renderer.state.level.CameraRenderState;
+
+public final class PlayerCosmeticRenderLayer extends RenderLayer<AvatarRenderState, PlayerModel> {
+    // Blockbench uses Minecraft model units around the vanilla body origin; no corrective transform is needed.
+    private static final float MODEL_SCALE = 1.0F;
+
+    private final PlayerCosmeticRenderer renderer = new PlayerCosmeticRenderer();
+
+    public PlayerCosmeticRenderLayer(RenderLayerParent<AvatarRenderState, PlayerModel> parent) {
+        super(parent);
+    }
+
+    @Override
+    public void submit(
+            PoseStack poseStack,
+            SubmitNodeCollector nodeCollector,
+            int packedLight,
+            AvatarRenderState state,
+            float limbAngle,
+            float limbDistance
+    ) {
+        CosmeticRenderData renderData = state.getData(CosmeticRenderStateKeys.COSMETIC);
+        CameraRenderState cameraRenderState = state.getData(CosmeticRenderStateKeys.CAMERA);
+
+        if (renderData == null || cameraRenderState == null || state.isInvisible || state.isSpectator
+                || !renderData.cosmetic().visible()) {
+            return;
+        }
+
+        PlayerCosmetic animatable = PlayerCosmeticManager.getOrCreateAnimatable(renderData.playerId());
+        poseStack.pushPose();
+
+        try {
+            getParentModel().body.translateAndRotate(poseStack);
+            poseStack.scale(MODEL_SCALE, MODEL_SCALE, MODEL_SCALE);
+            renderer.performRenderPass(
+                    animatable,
+                    renderData,
+                    poseStack,
+                    nodeCollector,
+                    cameraRenderState,
+                    packedLight,
+                    renderData.partialTick()
+            );
+        } finally {
+            poseStack.popPose();
+        }
+    }
+}

src/main/java/de/fullrisk/citrusDev/client/cosmetic/PlayerCosmeticRenderer.java

@@ -0,0 +1,26 @@
+package de.fullrisk.citrusDev.client.cosmetic;
+
+import com.geckolib.renderer.GeoObjectRenderer;
+import com.geckolib.renderer.base.GeoRenderState;
+import com.geckolib.renderer.base.RenderPassInfo;
+
+import java.util.UUID;
+
+public final class PlayerCosmeticRenderer extends GeoObjectRenderer<
+        PlayerCosmetic, CosmeticRenderData, GeoRenderState> {
+
+    public PlayerCosmeticRenderer() {
+        super(new PlayerCosmeticModel());
+    }
+
+    @Override
+    public long getInstanceId(PlayerCosmetic animatable, CosmeticRenderData data) {
+        UUID uuid = data.playerId();
+        return uuid.getMostSignificantBits() ^ uuid.getLeastSignificantBits();
+    }
+
+    @Override
+    public void adjustRenderPose(RenderPassInfo<GeoRenderState> renderPassInfo) {
+        // The layer already supplies a body-local pose; omit GeoObjectRenderer's block-centering offset.
+    }
+}