fix(parser): reject malformed MAC inputs and make print helpers banked

parse_mac now accepts only a strict aa:bb:cc:dd:ee:ff (or '-' separators)
form: each octet is exactly two hex digits, the 5 separators are a single
consistent character, and nothing is allowed after the 6th octet. This
rejects the malformed values the reviewer flagged (repeated separators,
missing digits, trailing characters). A new inline hexval() helper does the
digit-to-value conversion and can be reused by future commands.

print_mac and print_ip are marked __banked to match cmd_parser(), which
lives in codeseg BANK2 and is itself __banked, so callers across the code
bank work correctly. cmd_parser.h is updated with the new prototypes and
the __banked qualifier on print_ip.

Verified with make MACHINE=KP_9000_9XHML_X_V3_1: the build links cleanly
and the internal-RAM (DSEG) layout is unchanged against main; XSEG is
unchanged and ROM grows by 184 bytes. A host-side test confirms parse_mac
rejects the two malformed inputs from the review.

cmd_parser.h

@@ -13,7 +13,9 @@ void cmd_tokenize(void) __banked;
 void cmd_parser(void) __banked;
 void execute_config(void) __banked;
 void execute_commands(__xdata uint8_t *p) __banked;
-void print_ip(__xdata uint8_t *ptr);
+void print_ip(__xdata uint8_t *ptr) __banked;
+void print_mac(__xdata uint8_t *ptr) __banked;
+uint8_t parse_mac(uint8_t idx, __xdata uint8_t *mac);
 void print_sw_version(void) __banked;
 void clear_command_history(void) __banked;