neoforge-1.21.1: runtime fixes and build improvements

- Fix ServerTickEvent: register listener for ServerTickEvent.Post instead of
  abstract ServerTickEvent base class (NeoForge 21.x requirement)
- Fix null BlockGetter: replace null with EmptyBlockGetter.INSTANCE in
  isSolidRender() and propagatesSkylightDown() calls (1.21+ actually uses param)
- Fix chunk iteration: replace direct visibleChunkMap field access with
  getChunks() iteration and getChunkToSend() with getLatestChunk() (1.21.1 API)
- Build: fix shadowJar config to properly bundle DynmapCore into fat jar
  (was producing 80KB hollow jar missing all core classes)
- Build: cap Gradle daemon and forked javac heap to prevent OOM on large hosts
- Tested: server starts cleanly, Dynmap web UI accessible, maps rendered

build.gradle

@@ -30,10 +30,15 @@ minecraft {
     }
 }
 
+configurations {
+    shadow
+    implementation.extendsFrom shadow
+}
+
 dependencies {
     implementation 'net.neoforged:neoforge:21.1.219'
-    // DynmapCore and DynmapCoreAPI as local jars (build these separately)
-    implementation fileTree(dir: 'libs', include: ['*.jar'])
+    // DynmapCore bundled into the fat jar via shadow config
+    shadow fileTree(dir: 'libs', include: ['*.jar'])
 }
 
 processResources {
@@ -50,7 +55,13 @@ shadowJar {
     mergeServiceFiles()
 
     archiveBaseName = 'Dynmap'
+    archiveVersion = project.version
     archiveClassifier = 'neoforge-1.21.1'
+
+    // Exclude signature files that cause security exceptions
+    exclude 'META-INF/*.RSA'
+    exclude 'META-INF/*.SF'
+    exclude 'META-INF/*.DSA'
 }
 
 tasks.jar {
@@ -66,4 +77,11 @@ tasks.jar {
     }
 }
 
-build.dependsOn(jar)
+// Build the fat jar, not the thin one
+build.dependsOn(shadowJar)
+
+// Cap memory on forked javac process (prevents OOM on large NeoForge compile)
+tasks.withType(JavaCompile).configureEach {
+    options.fork = true
+    options.forkOptions.memoryMaximumSize = '3g'
+}