Replace 0xFFFFFFFF sentinel with explicit getOutboundTotal()

Instead of overloading getOutboundCount() with a magic sentinel value,
add a dedicated getOutboundTotal() method to the PacketManager interface
that returns the total queue size without time filtering.

This eliminates the fragile convention that caused the regression and
makes the two operations — time-filtered count vs total count —
explicitly separate in the API.

src/helpers/StatsFormatHelper.h

@@ -14,7 +14,7 @@ public:
       board.getBattMilliVolts(),
       ms.getMillis() / 1000,
       err_flags,
-      mgr->getOutboundCount(0xFFFFFFFF)
+      mgr->getOutboundTotal()
     );
   }