[MCFurry]

Parse the optional multiplexer indicator on SG_ lines (M, mN and mNM). Previously these lines did not match the signal regex and were silently dropped.

• Signal gains is_multiplexor and multiplexer_value; is_multiplexed is now set from the parsed indicator.
• Decoding reads the multiplexor's raw value and only emits the non multiplexed signals plus those matching that value.
• New parse_signals / parse_message overloads return values keyed by signal name, since positional output is ambiguous for multiplexed messages.

Extended multiplexing (SG_MUL_VAL_) is not supported yet: mNM signals are parsed, but all multiplexed signals are switched by the top level multiplexor.