Parser: fix exponential parse time on speculative NOT prefix parsing

[LucaCappelletti94]

parse_prefix wraps parse_expr_prefix_by_reserved_word in try_parse, which speculatively dispatches parse_not for the NOT keyword. parse_not's inner parse_subexpr(UnaryNot) infix-loops through -, hits the next NOT and recurses, so on chains like SELECT x-not-b.x-not-b... ending in a parse error every NOT segment re-walks the rest of the chain and parse_not is called 2^N - 1 times.

Fix is a per-parse BTreeSet<usize> of token positions where the speculative parse_not already failed. Subsequent visits at the same position skip the speculation and fall back to identifier interpretation.

Input	Before	After
903 B cursed libFuzzer artifact	~12 s	~2.5 ms
SELECT x-not-b. * 22	~11 s	~1 ms
SELECT x-not-b. * 30	(too slow to measure)	~1.5 ms