feat(bbmodel): 支持转换 Blockbench 方块模型

新增 bbmodel 子命令，将 .bbmodel 转成网易 netease:block_geometry，并支持独立输出目录。

同时导出内嵌 PNG 贴图并在资源包模式下更新 terrain_texture.json，减少手工导入方块模型和材质的重复操作。

Cargo.lock

@@ -86,6 +86,12 @@ dependencies = [
  "derive_arbitrary",
 ]
 
+[[package]]
+name = "base64"
+version = "0.22.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b3254f16251a8381aa12e40e3c4d2f0199f8c6508fbecb9d91f575e0fbb8c6"
+
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -308,6 +314,7 @@ dependencies = [
 name = "emod-cli"
 version = "0.1.0-dev"
 dependencies = [
+ "base64",
  "clap",
  "rand",
  "regex",