run.sh net: the per-device interface selection must PERSIST (an empty cfg dir produces no tap); -networkprovider alone does not choose one

harness/mame/run.sh

@@ -92,13 +92,16 @@ case "$MODE" in
         # the crash-work configuration: skip + tablet (already the default) + the LANCE
         # attached to a host tap.
         #
-        # A PRIVATE cfg directory is essential.  MAME stores the network provider PER
-        # DEVICE in cfg/<game>.cfg, and a saved value wins over -networkprovider -- a
-        # stale interface="-1" (i.e. "none") means no tap is ever opened, silently, with
-        # no verbose output.  Refresh it each run so a stale saved interface cannot
-        # reappear.  It needs the tap to exist and be owned by your user.
+        # A private cfg directory keeps the harness's network choice out of your
+        # ~/.mame, but note it must PERSIST: MAME stores the provider PER DEVICE in
+        # cfg/<game>.cfg, and -networkprovider only says which module may be used --
+        # it does NOT choose an interface.  With nothing saved, a network device
+        # defaults to "none" and no tap is ever opened (verified: an empty cfg dir
+        # produces no tap).  So choose it ONCE via MAME's UI while running this mode:
+        #   Tab -> Network Devices -> the LANCE -> taptun
+        # That selection is saved here and reused on every later `net` run.
         CFGDIR="$HERE/cfg-net"
-        rm -rf "$CFGDIR"; mkdir -p "$CFGDIR"
+        mkdir -p "$CFGDIR"
         exec env VT1200_SKIP_LEVEL1=1 \
             "$MAME" vt1200 "${COMMON[@]}" -cfg_directory "$CFGDIR" \
             -networkprovider taptun -window "$@"