Fix VLAN name persistence across rename and delete operations

Previously, renaming a VLAN or deleting and recreating it with a
different name did not update the displayed name. The vlan_names[]
array is an append-only buffer where vlan_name() returns the first
matching entry, so stale entries kept winning.

This commit adds vlan_name_remove(), which locates an entry by
VLAN ID and removes it via array compaction. The function is called
in two places:

  - parse_vlan() in cmd_parser.c, before appending a new name entry,
    to remove any pre-existing entry for the same VLAN ID
  - vlan_delete() in rtl837x_port.c, to clean up the name when a
    VLAN is removed

The implementation reuses the existing vlan_name() lookup, scans for
the trailing space of the matched entry, then shifts remaining bytes
left. Locals are declared as static __xdata to avoid the SDCC
overlay segment limit on banked functions.

Tested on KeepLiNK KP-9000-6XH-X:
  - vlan 99 AAA p1u; vlan 99 BBB -> name updated to BBB
  - vlan 99 d; vlan 99 CCC p1u   -> name correctly CCC, not stale AAA

Note: This fix addresses the runtime XMEM state. Persistence of
renamed VLAN names across reboot requires the user to download and
re-upload /config, as is the existing pattern for all configuration
changes in this firmware.

cmd_parser.c

@@ -347,6 +347,7 @@ void parse_vlan(void)
 		uint8_t w = 2;
 		if (cmd_words_len > w && isletter(cmd_buffer[cmd_words_b[w]])) {
 			register uint8_t i = 0;
+			vlan_name_remove(vlan_settings.vlan);
 			vlan_names[vlan_ptr++] = hex[(vlan_settings.vlan >> 8) & 0xf];
 			vlan_names[vlan_ptr++] = hex[(vlan_settings.vlan >> 4) & 0xf] ;
 			vlan_names[vlan_ptr++] = hex[vlan_settings.vlan & 0xf];