feat: add Engine.FeatureSupport.hasChecksums constant

Lets Lua code check whether Spring.GetPrevFrameChecksum() is available
without nil-guarding its return value, addressing PR review feedback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

rts/Lua/LuaConstEngine.cpp

@@ -28,6 +28,7 @@
  * @field noHandicapForReclaim boolean Whether handicap is applied to income from reclaim
  * @field groupAddDoesntSelect boolean Whether 'group add' also selects the group (does both if false)
  * @field deadTeamsKeepUnitLimit boolean Whether engine redistributes dead team unitlimit to allies (false) or keeps it as-is (true)
+ * @field hasChecksums boolean Whether the engine was built with SYNCCHECK (i.e. Spring.GetPrevFrameChecksum() returns a value)
  */
 
 /***
@@ -73,7 +74,7 @@ bool LuaConstEngine::PushEntries(lua_State* L)
 	 *
 	 * will be compatible even on engines that don't yet know about the entry at all. */
 	lua_pushliteral(L, "FeatureSupport");
-	lua_createtable(L, 0, 11);
+	lua_createtable(L, 0, 12);
 		LuaPushNamedBool(L, "NegativeGetUnitCurrentCommand", true);
 		LuaPushNamedBool(L, "hasExitOnlyYardmaps", true);
 		LuaPushNamedNumber(L, "rmlUiApiVersion", 1);
@@ -87,6 +88,11 @@ bool LuaConstEngine::PushEntries(lua_State* L)
 		LuaPushNamedBool(L, "noHandicapForReclaim", true);
 		LuaPushNamedBool(L, "groupAddDoesntSelect", true);
 		LuaPushNamedBool(L, "deadTeamsKeepUnitLimit", false);
+#ifdef SYNCCHECK
+		LuaPushNamedBool(L, "hasChecksums", true);
+#else
+		LuaPushNamedBool(L, "hasChecksums", false);
+#endif
 	lua_rawset(L, -3);
 
 	lua_pushliteral(L, "textColorCodes");

rts/Lua/LuaSyncedRead.cpp

@@ -946,7 +946,8 @@ int LuaSyncedRead::GetGameSeconds(lua_State* L)
  * built with `LUA_NUMBER = float`, which cannot represent a 32-bit
  * checksum losslessly.
  *
- * Returns `nil` on builds without `SYNCCHECK`.
+ * Returns `nil` on builds without `SYNCCHECK`. Check
+ * `Engine.FeatureSupport.hasChecksums` to detect availability.
  *
  * @function Spring.GetPrevFrameChecksum
  *