Add seed validation for server world creation and override-seed property

The dedicated server previously picked a completely random seed with no
biome diversity checks (the client validates but the server skipped it).
On top of that, the client's findSeed() was hardcoded to only check a
54-chunk (Classic) area, so Large worlds had no diversity guarantee
beyond the center.

New server worlds now use findSeed() scaled to the full target world
size. Added override-seed in server.properties to fix existing worlds
without deleting them.

Minecraft.Server/ServerProperties.cpp

@@ -824,6 +824,20 @@ ServerPropertiesConfig LoadServerPropertiesConfig()
 	config.maxPlayers = ReadNormalizedIntProperty(&merged, "max-players", kDefaultMaxPlayers, 1, kMaxDedicatedPlayers, &shouldWrite);
 	config.seed = 0;
 	config.hasSeed = ReadNormalizedOptionalInt64Property(&merged, "level-seed", &config.seed, &shouldWrite);
+	config.overrideSeed = 0;
+	config.hasOverrideSeed = false;
+	{
+		auto it = merged.find("override-seed");
+		if (it != merged.end() && !TrimAscii(it->second).empty())
+		{
+			__int64 parsed = 0;
+			if (TryParseInt64(TrimAscii(it->second), &parsed))
+			{
+				config.overrideSeed = parsed;
+				config.hasOverrideSeed = true;
+			}
+		}
+	}
 	config.logLevel = ReadNormalizedLogLevelProperty(&merged, "log-level", eServerLogLevel_Info, &shouldWrite);
 	config.autosaveIntervalSeconds = ReadNormalizedIntProperty(&merged, "autosave-interval", kDefaultAutosaveIntervalSeconds, 5, 3600, &shouldWrite);
 

Minecraft.Server/ServerProperties.h

@@ -31,6 +31,9 @@ namespace ServerRuntime
 		bool hasSeed;
 		/** `level-seed` */
 		__int64 seed;
+		/** `override-seed` replaces the seed for biome generation on existing worlds */
+		bool hasOverrideSeed;
+		__int64 overrideSeed;
 		/** `log-level` */
 		EServerLogLevel logLevel;
 		/** `autosave-interval` (seconds) */

Minecraft.Server/Windows64/ServerMain.cpp

@@ -28,6 +28,8 @@
 #include "../../Minecraft.World/TilePos.h"
 #include "../../Minecraft.World/compression.h"
 #include "../../Minecraft.World/OldChunkStorage.h"
+#include "../../Minecraft.World/BiomeSource.h"
+#include "../../Minecraft.World/LevelType.h"
 #include "../../Minecraft.World/net.minecraft.world.level.tile.h"
 #include "../../Minecraft.World/Random.h"
 
@@ -545,6 +547,12 @@ int main(int argc, char **argv)
 	app.SetGameNewHellScale(serverProperties.worldHellScale);
 #endif
 
+	if (serverProperties.hasOverrideSeed)
+	{
+		LogInfof("startup", "Seed override active: %lld", serverProperties.overrideSeed);
+		app.SetSeedOverride(serverProperties.overrideSeed);
+	}
+
 	StorageManager.SetSaveDisabled(serverProperties.disableSaving);
 	// Read world name and fixed save-id from server.properties
 	// Delegate load-vs-create decision to WorldManager
@@ -584,9 +592,17 @@ int main(int argc, char **argv)
 	{
 		param->seed = config.seed;
 	}
+	else if (worldBootstrap.saveData == nullptr)
+	{
+		// Only run seed validation when creating a brand-new world.
+		// Existing worlds already have their seed in level.dat.
+		LogInfof("startup", "Finding seed with biome diversity for %d-chunk world...", config.worldSizeChunks);
+		param->seed = BiomeSource::findSeed(LevelType::lvl_normal, config.worldSizeChunks);
+		LogInfof("startup", "Selected seed: %lld", param->seed);
+	}
 	else
 	{
-		param->seed = (new Random())->nextLong();
+		param->seed = (new Random())->nextLong(); // placeholder; level.dat seed takes priority
 	}
 #ifdef _LARGE_WORLDS
 	param->xzSize = (unsigned int)config.worldSizeChunks;