feat: Fabric 1.21.4 + Baritone 自动铺轨 mod 完整实现

- RailProcess: 点到点铺轨（Yaw 取向前方 2 格、合法右键放置、GoalBlock 行走循环）
- 节奏 5 tick(250ms) 适配多人服务器、热键栏切换与 UpdateSelectedSlot 同步
- 悬空自动用圆石垫脚石搭桥、calcFailed 卡死兜底
- 修复 1.21.4 sendMessage 签名编译错误、Gradle 守护进程钉到 JDK 21
- 新增 README 与 .gitignore

build.gradle

@@ -0,0 +1,41 @@
+plugins {
+    id 'fabric-loom' version '1.9-SNAPSHOT'
+    id 'maven-publish'
+}
+
+version = project.mod_version
+group = project.maven_group
+
+base {
+    archivesName = project.archives_base_name
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    minecraft "com.mojang:minecraft:${project.minecraft_version}"
+    mappings "net.fabricmc:yarn:${project.yarn_mappings}:v2"
+    modImplementation "net.fabricmc:fabric-loader:${project.loader_version}"
+    modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
+
+    modImplementation files("libs/baritone-api-fabric-${project.baritone_version}.jar")
+}
+
+processResources {
+    inputs.property "version", project.version
+    filesMatching("fabric.mod.json") {
+        expand "version": project.version
+    }
+}
+
+tasks.withType(JavaCompile).configureEach {
+    it.options.encoding = "UTF-8"
+    it.options.release = 21
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
+}