active_parameter out of range for first param of multi-word instructions

[aazuspan]

When getting the arg index for signature help on multi-word instructions, we offset the index by -1 to account for the extra arg:

spinasm-lsp/src/spinasm_lsp/server.py

Lines 359 to 360 in 4d6e946

	if str(triggered_opcode) in MULTI_WORD_INSTRUCTIONS and arg_idx != -1:
	arg_idx -= 1

On the first parameter, this makes the arg index -1 which is out of range for SignatureHelp.active_parameter, leading to a ValueError. The offset should only be applied if arg_idx != 0. The existing check for arg_idx != -1 can be removed, since that assumed that -1 would correspond with the final parameter.