Restore natural mob spawning

- Restore cave spawning and correct floor placement across platforms.
- Enforce category population limits and clamp ambient spawn batches.
- Honor mob-spawning rules and entity-ticking chunk admission.
- Separate chunk saving from configurable distance-despawn retention.
- Add spawning regressions and Folia gameplay acceptance coverage.

adapters/bukkit/plugin/src/test/gameplay/natural-spawning-fixture/build.gradle

@@ -0,0 +1,35 @@
+plugins {
+    id 'java'
+}
+
+version = '1.0.0'
+
+repositories {
+    mavenCentral()
+    maven { url = 'https://repo.papermc.io/repository/maven-public/' }
+}
+
+dependencies {
+    compileOnly files(providers.gradleProperty('irisJar').get())
+    compileOnly files(providers.gradleProperty('irisGsonJar').get())
+    compileOnly 'io.papermc.paper:paper-api:26.1.2.build.74-stable'
+}
+
+java {
+    toolchain.languageVersion = JavaLanguageVersion.of(25)
+}
+
+sourceSets {
+    main {
+        java.srcDirs = ['src']
+        resources.srcDirs = ['resources']
+    }
+}
+
+tasks.withType(JavaCompile).configureEach {
+    options.compilerArgs += '-parameters'
+}
+
+tasks.named('jar') {
+    archiveBaseName = 'IrisNaturalSpawningFixture'
+}