feat: Add GetPrevFrameChecksum() to the Lua API

# Purpose
Trying to expose the sim frame checksum information to lua so we can update the fightertest widget/gadget combo to grab it every sim frame and dump it to a file at the end of each fightertest run.

Author: bruno-dasilva

rts/Lua/LuaSyncedRead.cpp

@@ -79,6 +79,7 @@
 #include "System/FileSystem/FileHandler.h"
 #include "System/FileSystem/FileSystem.h"
 #include "System/StringUtil.h"
+#include "System/Sync/SyncChecker.h"
 
 #include <cctype>
 #include <functional>
@@ -121,6 +122,7 @@ bool LuaSyncedRead::PushEntries(lua_State* L)
 
 	REGISTER_LUA_CFUNC(GetGameFrame);
 	REGISTER_LUA_CFUNC(GetGameSeconds);
+	REGISTER_LUA_CFUNC(GetPrevFrameChecksum);
 
 	REGISTER_LUA_CFUNC(GetGameRulesParam);
 	REGISTER_LUA_CFUNC(GetGameRulesParams);
@@ -931,6 +933,38 @@ int LuaSyncedRead::GetGameSeconds(lua_State* L)
 }
 
 
+/***
+ *
+ * Returns the engine's final sync checksum for the previous simulation
+ * frame as an 8-character lowercase hex string (e.g. `"fade1eaf"`).
+ * The value is stable for the entire duration of the current frame
+ * (independent of gadget/callin ordering) and is identical across all
+ * in-sync clients, so it can be folded into a deterministic per-run
+ * artifact for sync testing (e.g. fightertest).
+ *
+ * Returned as a string rather than a number because Spring's Lua is
+ * built with `LUA_NUMBER = float`, which cannot represent a 32-bit
+ * checksum losslessly.
+ *
+ * Returns `nil` on builds without `SYNCCHECK`.
+ *
+ * @function Spring.GetPrevFrameChecksum
+ *
+ * @return string|nil checksum
+ */
+int LuaSyncedRead::GetPrevFrameChecksum(lua_State* L)
+{
+#ifdef SYNCCHECK
+	char buf[16];
+	snprintf(buf, sizeof(buf), "%08x", CSyncChecker::GetPrevChecksum());
+	lua_pushstring(L, buf);
+#else
+	lua_pushnil(L);
+#endif
+	return 1;
+}
+
+
 /******************************************************************************
  * Environment
  * @section environment

rts/Lua/LuaSyncedRead.h

@@ -45,6 +45,7 @@ class LuaSyncedRead {
 
 		static int GetGameFrame(lua_State* L);
 		static int GetGameSeconds(lua_State* L);
+		static int GetPrevFrameChecksum(lua_State* L);
 
 		static int GetGameRulesParam(lua_State* L);
 		static int GetGameRulesParams(lua_State* L);

rts/Net/NetCommands.cpp

@@ -623,6 +623,12 @@ void CGame::ClientReadNet()
 				ASSERT_SYNCED(CSyncChecker::GetChecksum());
 				clientNet->Send(CBaseNetProtocol::Get().SendSyncResponse(gu->myPlayerNum, gs->frameNum, CSyncChecker::GetChecksum()));
 
+				// Cache the just-sent, frame-closed checksum so Lua gadgets
+				// running in the NEXT sim frame can read a stable value via
+				// Spring.GetPrevFrameChecksum(). Must happen before the
+				// 4096-frame reset below so we capture the pre-reset value.
+				CSyncChecker::SetPrevChecksum(CSyncChecker::GetChecksum());
+
 				// buffer all checksums, so we can check sync later between demo & local
 				if (haveServerDemo)
 					localSyncChecksums[gs->frameNum] = CSyncChecker::GetChecksum();

rts/System/Sync/SyncChecker.cpp

@@ -11,6 +11,7 @@
 
 
 unsigned CSyncChecker::g_checksum;
+unsigned CSyncChecker::g_prevChecksum;
 int CSyncChecker::inSyncedCode;
 
 void CSyncChecker::NewFrame()

rts/System/Sync/SyncChecker.h

@@ -31,6 +31,8 @@ class CSyncChecker {
 		 * Keeps a running checksum over all assignments to synced variables.
 		 */
 		static unsigned GetChecksum() { return g_checksum; }
+		static unsigned GetPrevChecksum() { return g_prevChecksum; }
+		static void SetPrevChecksum(unsigned v) { g_prevChecksum = v; }
 		static void NewFrame();
 		static void debugSyncCheckThreading();
 		static void Sync(uint32_t val);
@@ -48,6 +50,11 @@ class CSyncChecker {
 		 */
 		static unsigned g_checksum;
 
+		/**
+		 * Final checksum of the previous simulation frame.
+		 */
+		static unsigned g_prevChecksum;
+
 		/**
 		 * @brief in synced code
 		 *