feat: 补全 /place structure 的结构注册，使其与自然生成一致

原版 PlaceCommand.placeStructure 只调 StructureStart.placeInChunk 摆放方块，从不把 StructureStart 注册进 StructureManager / 区块 StructureAccess，导致结构重启丢失、专属刷怪规则不激活。

新增 PlaceCommandMixin，在 placeStructure 返回前补做自然生成的三步：1) setStartForStructure 把 start 存进所在区块；2) createReferences 写 17x17 邻居引用；3) markUnsaved 强制存盘。全程 try-catch 兜底，注册失败不影响命令返回。

项目骨架参照 CarpetSLSAddition：Loom 1.18-SNAPSHOT + Gradle 9.7.1 + Java 25 + official 映射。

src/main/java/com/github/axtps/placemod/mixin/PlaceCommandMixin.java

@@ -0,0 +1,88 @@
+package com.github.axtps.placemod.mixin;
+
+import com.github.axtps.placemod.PlaceMod;
+import net.minecraft.core.SectionPos;
+import net.minecraft.server.commands.PlaceCommand;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.level.ChunkPos;
+import net.minecraft.world.level.StructureManager;
+import net.minecraft.world.level.chunk.ChunkAccess;
+import net.minecraft.world.level.chunk.ChunkGenerator;
+import net.minecraft.world.level.levelgen.structure.BoundingBox;
+import net.minecraft.world.level.levelgen.structure.Structure;
+import net.minecraft.world.level.levelgen.structure.StructureStart;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+import org.spongepowered.asm.mixin.injection.callback.LocalCapture;
+
+/**
+ * 让 /place structure 生成的结构"跟自然生成一样"。
+ *
+ * 原版 PlaceCommand.placeStructure 只调了 StructureStart.placeInChunk 摆放方块，
+ * 从不把 StructureStart 注册进 StructureManager / 区块 StructureAccess。
+ * 结果：结构不进 StructureManager、不写区块 NBT 引用 → 重启丢失、专属刷怪规则不激活。
+ *
+ * 本 Mixin 在 placeStructure 返回前，补做自然生成的注册步骤：
+ *   1. 对结构所在区块 setStartForStructure，把 start 存进区块；
+ *   2. 调 ChunkGenerator.createReferences 写周围区块引用（跟自然生成同款逻辑）；
+ *   3. markUnsaved 标记涉及的区块，强制存盘。
+ */
+@Mixin(PlaceCommand.class)
+public class PlaceCommandMixin {
+
+	@Inject(method = "placeStructure", at = @At("RETURN"), locals = LocalCapture.CAPTURE_FAILHARD)
+	private static void placemod$registerStructure(net.minecraft.commands.CommandSourceStack source,
+												   net.minecraft.core.Holder.Reference<Structure> structureHolder,
+												   net.minecraft.core.BlockPos pos,
+												   CallbackInfo ci,
+												   ServerLevel level,
+												   Structure structure,
+												   ChunkGenerator generator,
+												   StructureStart start) {
+		if (start == null || !start.isValid()) {
+			return;
+		}
+
+		try {
+			StructureManager structureManager = level.structureManager();
+			BoundingBox box = start.getBoundingBox();
+
+			// 结构包围盒覆盖的区块范围
+			ChunkPos minChunk = new ChunkPos(
+					SectionPos.blockToSectionCoord(box.minX()),
+					SectionPos.blockToSectionCoord(box.minZ()));
+			ChunkPos maxChunk = new ChunkPos(
+					SectionPos.blockToSectionCoord(box.maxX()),
+					SectionPos.blockToSectionCoord(box.maxZ()));
+
+			// 1) 把 StructureStart 存进它自己所在的区块
+			//    （自然生成在 ChunkGenerator.createStructures 里做）
+			ChunkPos startChunk = start.getChunkPos();
+			ChunkAccess startChunkAccess = level.getChunk(startChunk.x(), startChunk.z());
+			structureManager.setStartForStructure(
+					SectionPos.bottomOf(startChunkAccess), structure, start, startChunkAccess);
+			startChunkAccess.markUnsaved();
+
+			// 2) 引用写入：对涉及的每个区块调 createReferences，
+			//    它内部会扫 17x17 邻居，把相交的 StructureStart 引用写进区块。
+			//    跟自然生成用的是同一个方法，保证"完全一样"。
+			ChunkPos.rangeClosed(minChunk, maxChunk).forEach(cp -> {
+				ChunkAccess chunk = level.getChunk(cp.x(), cp.z());
+				try {
+					generator.createReferences(level, structureManager, chunk);
+					chunk.markUnsaved();
+				} catch (Exception e) {
+					PlaceMod.LOGGER.error("[place-mod] createReferences 失败 @ {}: {}", cp, e.toString());
+				}
+			});
+
+			PlaceMod.LOGGER.info("[place-mod] 已为结构 {} 补注册 StructureStart @ {}",
+					structure, startChunk);
+		} catch (Throwable t) {
+			// 注册失败不能影响命令本身的成功返回
+			PlaceMod.LOGGER.error("[place-mod] 补注册结构时出错（已忽略）: ", t);
+		}
+	}
+}