Fix: Don't cache empty tiles to disk

Empty tiles (~270 bytes) are now detected and skipped from caching.
This fixes the issue where unexplored chunks would cache empty tiles,
and then continue serving stale empty tiles even after the chunk was explored.

- Added EMPTY_TILE_THRESHOLD = 500 bytes
- Skip caching for tiles below threshold in all cache paths
- Ensures newly explored chunks regenerate fresh tiles

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cryptobench

src/main/java/com/easywebmap/map/TileManager.java

@@ -28,6 +28,8 @@ public class TileManager {
     // Limit concurrent tile generations to prevent CPU spikes
     private static final int MAX_CONCURRENT_GENERATIONS = 4;
     private final Semaphore generationSemaphore = new Semaphore(MAX_CONCURRENT_GENERATIONS);
+    // Empty tiles are ~270 bytes, real tiles are 10KB+
+    private static final int EMPTY_TILE_THRESHOLD = 500;
 
     public TileManager(EasyWebMap plugin) {
         this.plugin = plugin;
@@ -100,7 +102,7 @@ private CompletableFuture<byte[]> getCompositeTile(String worldName, int zoom, i
         this.pendingRequests.put(cacheKey, future);
         future.whenComplete((data, ex) -> {
             this.pendingRequests.remove(cacheKey);
-            if (data != null && data.length > 0 && ex == null) {
+            if (data != null && data.length > EMPTY_TILE_THRESHOLD && ex == null) {
                 this.memoryCache.put(cacheKey, data);
                 if (this.plugin.getConfig().isUseDiskCache()) {
                     this.diskCache.putAsync(worldName, zoom, tileX, tileZ, data);
@@ -159,7 +161,8 @@ public CompletableFuture<byte[]> getBaseTile(String worldName, int tileX, int ti
         this.pendingRequests.put(cacheKey, future);
         future.whenComplete((data, ex) -> {
             this.pendingRequests.remove(cacheKey);
-            if (data != null && data.length > 0 && ex == null) {
+            // Don't cache empty tiles - they should regenerate when chunk gets explored
+            if (data != null && data.length > EMPTY_TILE_THRESHOLD && ex == null) {
                 this.memoryCache.put(cacheKey, data);
                 if (this.plugin.getConfig().isUseDiskCache()) {
                     this.diskCache.putAsync(worldName, 0, tileX, tileZ, data);
@@ -193,7 +196,8 @@ public CompletableFuture<PngEncoder.TileData> getBaseTileWithPixels(String world
         this.pendingPixelRequests.put(cacheKey, future);
         future.whenComplete((data, ex) -> {
             this.pendingPixelRequests.remove(cacheKey);
-            if (data != null && !data.isEmpty() && ex == null) {
+            // Don't cache empty tiles - they should regenerate when chunk gets explored
+            if (data != null && !data.isEmpty() && data.pngBytes.length > EMPTY_TILE_THRESHOLD && ex == null) {
                 // Cache pixels for compositing, evict if too many
                 if (this.pixelCache.size() < MAX_PIXEL_CACHE) {
                     this.pixelCache.put(cacheKey, data);