play_solo.bat: single-player practice launcher in the player export

A zip recipient can dork around offline -- no -net argument means the
game self-launches a solo mission (all maps ship in content/).  Solo
gets its own exit message (btl4.log pointer, no server language).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

players/play_solo.bat

@@ -0,0 +1,11 @@
+@echo off
+rem BT411 -- single-player practice mission (no network,
+rem no operator needed).  Edit the -egg name below to try
+rem other maps (ARENA1/ARENA2/DBASE/FOGDAY/...).
+set BT_START_INSIDE=1
+cd /d %~dp0content
+..\build\Release\btl4.exe -egg ARENA1.EGG
+echo.
+echo The game has exited.  If it closed unexpectedly,
+echo send the operator the file content\btl4.log.
+pause

tools/btoperator.py

@@ -729,10 +729,25 @@ class Operator(QMainWindow):
                     "cd /d %%~dp0content\n"
                     "..\\build\\Release\\btl4.exe -egg %s -net 1501\n"
                     % egg_name + tail)
+        # solo practice: no server involved -- the game self-launches a
+        # single-player mission (no -net argument = no console wait)
+        with open(os.path.join(out_dir, "play_solo.bat"), "w",
+                  newline="\r\n") as f:
+            f.write("@echo off\n"
+                    "rem BT411 -- single-player practice mission (no network,\n"
+                    "rem no operator needed).  Edit the -egg name below to try\n"
+                    "rem other maps (ARENA1/ARENA2/DBASE/FOGDAY/...).\n"
+                    "set BT_START_INSIDE=1\n"
+                    "cd /d %~dp0content\n"
+                    "..\\build\\Release\\btl4.exe -egg ARENA1.EGG\n"
+                    "echo.\n"
+                    "echo The game has exited.  If it closed unexpectedly,\n"
+                    "echo send the operator the file content\\btl4.log.\n"
+                    "pause\n")
         self.log.appendPlainText(
-            "exported join.bat + join_lan.bat to %s (public host: %s) -- "
-            "one file for everyone; the relay assigns seats"
-            % (out_dir, host))
+            "exported join.bat + join_lan.bat + play_solo.bat to %s "
+            "(public host: %s) -- join bats are universal; the relay "
+            "assigns seats" % (out_dir, host))
 
     # ------------------------------------------------------------- closing --