ci: multi-version build matrix + multi-MC compat layer

- Parametrize build: -PmcVersion / -PapiCoordinate / -PjavaRelease / -PapiVersion;
  jar name includes mc version, plugin.yml api-version parametrized
- Compat (reflection): sendBlockChanges / getClientViewDistance /
  getStructureManager (Paper 1.19.3+); particle()/potionEffect() name lookup
  for 1.21 renames (BLOCK_CRACK->BLOCK, REDSTONE->DUST, FIREWORKS_SPARK->
  FIREWORK, JUMP->JUMP_BOOST, SLOW->SLOWNESS); drop CHERRY materials (1.20+)
- Fix Java 17 'unconditional instanceof' in JoinCommandExecutor
- capabilitiesResolution for org.bukkit:bukkit capability conflict on newer
  paper-api artifacts
- .github/workflows/build.yml: matrix for 1.19.2/1.20.1 (Java 17),
  1.21/1.21.11 (Java 21), 26.1.2/26.2 (Java 25) + quality gate
- Verified locally: default + 1.19.2/1.20.1/1.21/1.21.11 build; 26.x deps resolve

build.gradle.kts

@@ -10,15 +10,16 @@ group = "dev.loki"
 version = "1.3.3"
 description = "LoParkour - Advanced parkour plugin for Minecraft"
 
-java {
-    toolchain {
-        languageVersion = JavaLanguageVersion.of(21)
-    }
-}
+// Multi-version build: pass -PmcVersion / -PapiCoordinate / -PjavaRelease / -PapiVersion
+// to build the same source against a different Paper API (e.g. GitHub Actions matrix).
+val mcVersion: String = providers.gradleProperty("mcVersion").getOrElse("1.20.4")
+val apiCoordinate: String = providers.gradleProperty("apiCoordinate")
+    .getOrElse("org.spigotmc:spigot-api:$mcVersion-R0.1-SNAPSHOT")
+val javaRelease: Int = providers.gradleProperty("javaRelease").map { it.toInt() }.getOrElse(21)
+val apiVersion: String = providers.gradleProperty("apiVersion").getOrElse("1.20")
 
 repositories {
     mavenCentral()
-    
     maven {
         name = "papermc"
         url = uri("https://repo.papermc.io/repository/maven-public/")
@@ -76,9 +77,29 @@ repositories {
     }
 }
 
+// Resolve the org.bukkit:bukkit capability conflict that newer paper-api artifacts
+// introduce (they both bundle the bukkit classes and declare the capability).
+configurations.configureEach {
+    resolutionStrategy.capabilitiesResolution.withCapability("org.bukkit:bukkit") {
+        selectHighestVersion()
+    }
+}
+
 dependencies {
-    // Spigot API
-    compileOnly("org.spigotmc:spigot-api:1.20.4-R0.1-SNAPSHOT")
+    // Spigot/Paper API (overridable per MC version via -PapiCoordinate).
+    // Newer paper-api artifacts declare a conflicting org.bukkit:bukkit capability
+    // (they bundle the classes themselves) — drop the duplicate transitive.
+    if (apiCoordinate.startsWith("io.papermc.paper:paper-api")) {
+        val (apiGroup, apiName, apiVersion) = apiCoordinate.split(":")
+        compileOnly(mapOf(
+            "group" to apiGroup,
+            "name" to apiName,
+            "version" to apiVersion,
+            "exclude" to mapOf("group" to "org.bukkit", "module" to "bukkit")
+        ))
+    } else {
+        compileOnly(apiCoordinate)
+    }
     
     // Adventure API (for Component)
     compileOnly("net.kyori:adventure-api:4.14.0")
@@ -116,7 +137,7 @@ dependencies {
 
 tasks.shadowJar {
     archiveClassifier.set("")
-    archiveFileName.set("LoParkour-${project.version}.jar")
+    archiveFileName.set("LoParkour-${project.version}-mc${mcVersion}.jar")
     
     // Relocate shaded dependencies
     relocate("dev.lolib", "dev.loki.loparkour.lib.lolib")
@@ -141,7 +162,8 @@ tasks.withType<ProcessResources> {
     filesMatching(listOf("plugin.yml", "config.yml")) {
         expand(
             "version" to project.version,
-            "description" to project.description
+            "description" to project.description,
+            "apiVersion" to apiVersion
         )
     }
 }
@@ -152,7 +174,7 @@ tasks.withType<Test> {
 
 tasks.withType<JavaCompile> {
     options.encoding = "UTF-8"
-    options.release = 21
+    options.release = javaRelease
 }
 
 tasks.withType<Javadoc> {