[Feature] fuzzy_check: anchor probability weight curve at the match threshold

[Feature] fuzzy_check: anchor probability weight curve at the match threshold

The score multiplier for non-exact matches was sqrt(prob): a concave
curve anchored at zero. Since the storage never returns prob below the
shingle match threshold (0.5), the whole reachable range collapsed into
(~0.73 .. 1.0], so a marginal 17/32 shingle overlap scored almost as
high as an exact match; combined with high-weight deny lists this
turned weak matches into instant rejects.

Replace it with ((prob - prob_bias) / (1 - prob_bias)) ^ prob_power,
anchored at the match threshold (prob_bias 0.5). prob_power defaults to
1.0: the multiplier is the fraction of the way from the threshold to an
exact match (19/32 -> 0.19, 24/32 -> 0.50, 28/32 -> 0.75); raise it per
rule for harsher discounting of weak matches. The old zero-anchored
sqrt curve is not preserved: it was simply broken. Applies to text and
HTML hashes; small images keep their existing normalized curve.