Steam gate: rejected players now get a MESSAGE BOX, not a silent quit

A failed join returns 1 to the FE, which QUITS the exe -- so every
rejection so far was a log line plus "the game just closed" (#68's exact
complaint). New LobbyNotice() = same text in the day log (flattened, still
greppable) + blocking MessageBox. Wired to every join-side bail:

- BUILD MISMATCH: when the version-filtered search is empty, probe once
  without the version filter; if a lobby IS up, the box names the host's
  build vs ours (lobby data rides the list result -- no join needed).
  Post-entry verify mismatch gets the same box.
- NO LOBBY FOUND: probe empty too -> plain no-lobby box naming our build.
- STEAM UNAVAILABLE: transport install failed.
- LEFT BEHIND: host launched without us (no token in btl4map).

Old exes still exit silently on rejection -- nothing shipped today can
add text to a binary players already have; the host's roster marker +
REJECT log line remain the operator's view of those.

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

game/glass/btl4lobby.cpp

@@ -49,6 +49,50 @@ static void
 }
 #include <stdarg.h>
 
+//
+// A rejection the PLAYER must see goes through LobbyNotice: the same text
+// lands in the day log (newlines flattened so the line stays greppable)
+// AND in a blocking message box.  These paths are synchronous FE flows --
+// a bare return here QUITS the exe, and #68 taught us a player bounced
+// with only a log line reports "the game just closed".
+//
+static void
+	LobbyNotice(const char *format, ...)
+{
+	char text[512];
+	va_list arguments;
+	va_start(arguments, format);
+	_vsnprintf(text, sizeof(text) - 1, format, arguments);
+	text[sizeof(text) - 1] = 0;
+	va_end(arguments);
+	{
+		char flat[512];
+		int n = 0;
+		for (const char *s = text; *s && n < 510; ++s)
+		{
+			if (*s == '\n')
+			{
+				if (n > 0 && flat[n - 1] == ' ')
+					continue;
+				flat[n++] = ' ';
+			}
+			else
+			{
+				flat[n++] = *s;
+			}
+		}
+		flat[n] = 0;
+		LobbyLog("NOTICE: %s", flat);
+	}
+	WCHAR wide[512];
+	int n = 0;
+	for (const char *s = text; *s && n < 511; ++s)
+		wide[n++] = (WCHAR)*s;
+	wide[n] = 0;
+	MessageBoxW(NULL, wide, L"BATTLETECH -- STEAM LOBBY",
+		MB_OK | MB_ICONWARNING | MB_SETFOREGROUND);
+}
+
 //###########################################################################
 // Synchronous Steam call-result helper (manual polling -- no callback
 // template machinery in this C-style TU).
@@ -458,7 +502,9 @@ int
 {
 	if (BTSteamNet_Install() != 0 || !BTSteamNet_Active())
 	{
-		LobbyLog("join: Steam transport unavailable");
+		LobbyNotice("STEAM UNAVAILABLE\n\n"
+			"Could not reach Steam.\n"
+			"Is Steam running and logged in?");
 		return -1;
 	}
 
@@ -477,9 +523,36 @@ int
 		LobbyMatchList_t::k_iCallback, 15000) != 0 ||
 		match_list.m_nLobbiesMatching == 0)
 	{
-		LobbyLog("join: no btl4 lobby found for build %s "
+		//
-			"(a lobby on a DIFFERENT build is invisible by design -- "
+		// Nothing on OUR build.  Probe once WITHOUT the version filter so
-			"host and joiners must run the same zip)", BT_VERSION_STRING);
+		// the rejection is specific: "the host is on a different zip"
+		// beats "no lobby" when one is in fact up.  Lobby data rides back
+		// with the list result, readable without joining.  (Filters only
+		// apply to the next request, so the probe re-adds the game key.)
+		//
+		matchmaking->AddRequestLobbyListStringFilter(
+			"btl4", "1", k_ELobbyComparisonEqual);
+		call = matchmaking->RequestLobbyList();
+		memset(&match_list, 0, sizeof(match_list));
+		if (WaitApiCall(call, &match_list, sizeof(match_list),
+			LobbyMatchList_t::k_iCallback, 8000) == 0 &&
+			match_list.m_nLobbiesMatching > 0)
+		{
+			const char *host_ver = matchmaking->GetLobbyData(
+				matchmaking->GetLobbyByIndex(0), "btl4ver");
+			LobbyNotice("BUILD MISMATCH\n\n"
+				"A lobby is up, but the host runs build %s\n"
+				"and this machine runs build %s.\n\n"
+				"Everyone must run the same zip to play together.",
+				(host_ver && *host_ver) ? host_ver : "(an older build)",
+				BT_VERSION_STRING);
+		}
+		else
+		{
+			LobbyNotice("NO LOBBY FOUND\n\n"
+				"No BattleTech lobby is up right now.\n"
+				"(This machine runs build %s.)", BT_VERSION_STRING);
+		}
 		return -1;
 	}
 
@@ -502,12 +575,14 @@ int
 		const char *host_ver = matchmaking->GetLobbyData(currentLobby, "btl4ver");
 		if (host_ver == NULL || strcmp(host_ver, BT_VERSION_STRING) != 0)
 		{
-			LobbyLog("join: BUILD MISMATCH -- host runs %s, this machine runs %s; "
-				"leaving (both sides must run the same zip)",
-				(host_ver && *host_ver) ? host_ver : "<unstamped/older>",
-				BT_VERSION_STRING);
 			matchmaking->LeaveLobby(currentLobby);
 			currentLobby = CSteamID();
+			LobbyNotice("BUILD MISMATCH\n\n"
+				"The host runs build %s\n"
+				"and this machine runs build %s.\n\n"
+				"Everyone must run the same zip to play together.",
+				(host_ver && *host_ver) ? host_ver : "(an older build)",
+				BT_VERSION_STRING);
 			return -1;
 		}
 	}
@@ -548,7 +623,15 @@ int
 		LobbyLog("join: my token [%s], map [%s]", my_token_out, steam_map_out);
 		if (my_token_out[0] == 0)
 		{
-			result = -1;			// the host's map is missing us
+			// The host launched without us -- no seat in the mission map.
+			// (Same-build clients can only hit this via a full lobby or a
+			// publish race now; version skew is gated before entry.)
+			LobbyNotice("LEFT BEHIND\n\n"
+				"The host launched the mission without this machine\n"
+				"(no seat in the mission map).\n\n"
+				"Rejoin on the next launch.  If this repeats, compare\n"
+				"builds: this machine runs %s.", BT_VERSION_STRING);
+			result = -1;
 		}
 	}
 	SteamMatchmaking()->LeaveLobby(currentLobby);