feat(ugc): answer the client's UGC manifest requests without 3D services

With UGCUSE3DSERVICES=7:0 (the client's default) the client asks its world for
a blueprint file's MD5 and size (REQUEST_UGC_MANIFEST_INFO, world 27) and then
downloads BrickModels/UserMade/<id % 1000>/<id>.<ext>.sd0. Layouts checked in
the 1.10.64 client: the request is a u64 blueprint id and a u8 resource type;
the answer (UGC_MANIFEST_RESPONSE, client 60) repeats them and adds a u8 valid,
the u32 size and the 16 byte MD5 of the inflated file, 37 bytes after the 0x53
exactly or the client drops it.

- dNet: WorldPackets::RequestUgcManifestInfo, ClientPackets::UgcManifestResponse
  (eUgcResourceType), with byte tests against the client's layouts.
- Database: ugc_file_checksums (per model or module combination and file) and
  ugc_modular_build.combination_id (migrations 89 / 72), GetUgcFileChecksum
  looks a blueprint up as a model, else as a build through its combination.
- UGC server: every download is also written as .sd0 (Sd0::Compress); workers
  hand the checksums back and the main thread stores them; old items get their
  sd0 icon and checksum, and builds their combination id, once at start-up, a
  few per tick; serves <dir>/BrickModels/UserMade/<bucket>/<id>.<ext>.sd0 under
  client_path, /<folder>/UserBrickModels and the root (.hkx 404).
- World: UgcManifest answers on the main thread with one indexed query per
  request; files not made yet are answered when they are (looked at again
  every 5 seconds), and a model in its quiet period is made right away. No
  worker threads, HTTP or file reads in the world.

Off by default (ugc_manifest=0): checked in game, the client then downloads
from http://127.0.0.1:80/lwoclient/UserBrickModels/ whatever its boot.cfg says
and logs the player out when it can't connect, so icons need the UGC server on
port 80 of each player's machine. docs/UgcServer.md has the details.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dNet/ClientPackets.h

@@ -6,6 +6,7 @@
 #ifndef CLIENTPACKETS_H
 #define CLIENTPACKETS_H
 
+#include <array>
 #include <cstdint>
 #include <ctime>
 #include <set>
@@ -18,6 +19,7 @@
 #include "eAddFriendResponseType.h"
 #include "eAddIgnoreResponse.h"
 #include "eBlueprintSaveResponseType.h"
+#include "eUgcResourceType.h"
 #include "MessageType/Client.h"
 #include "MessageType/Game.h"
 #include "NiPoint3.h"
@@ -264,6 +266,23 @@ namespace ClientPackets {
 		LWOOBJID itemId{};
 		LWOOBJID destItemId{};
 	};
+
+	// Server -> client. The answer to REQUEST_UGC_MANIFEST_INFO: the MD5 and size of one of a blueprint's files as the
+	// client has it after downloading and inflating it (BrickModels/UserMade/<id % 1000>/<id>.<ext>.sd0 from the UGC
+	// server). The client checks a file it has against it and downloads the file when it differs; with valid 0 it uses
+	// a file it has as it is and downloads it only when it has none. The client ignores the answer unless it is exactly
+	// this long (37 bytes after the 0x53).
+	struct UgcManifestResponse : public LUBitStream {
+		UgcManifestResponse() : LUBitStream(ServiceType::CLIENT, MessageType::Client::UGC_MANIFEST_RESPONSE) {}
+		void Serialize(RakNet::BitStream& bitStream) const override;
+		bool Deserialize(RakNet::BitStream& bitStream) override;
+
+		LWOOBJID blueprintId{};
+		eUgcResourceType resourceType{};
+		bool valid{}; // A byte
+		uint32_t fileSize{}; // Of the inflated file
+		std::array<uint8_t, 16> md5{}; // Of the inflated file, as bytes
+	};
 };
 
 /**