Stop comments from starting multiline values - #615

Comment recognition happens only after multiline detection, and the multiline heuristic searches the whole physical line for a quote start, so a comment containing the characters =" opened a phantom multiline buffer: every following line was silently swallowed until a stray closing quote arrived or the buffer was discarded at end of file, making unrelated later assignments vanish without an error, including for realistic inputs like a URL value followed by a comment mentioning a quoted example. The multiline start is now ignored when a hash appears earlier in the line than the quote start, so comments can no longer open multiline values, while genuine multiline values, including ones whose opening line contains a hash inside the quotes, are unchanged. The only behaviour change on previously accepted input is that a stray quote line following such a comment now fails name validation loudly instead of being silently dropped with its neighbours. This is stacked on #608.