The candidate matcher rejected valid downloads of titles with a '/' or ':' (e.g.
Sawano's "You See Big Girl / T:T") because the unified normalize() removed those
chars ("t:t" -> "tt") while keeping the '_' that source filenames substitute for
them ("T_T" -> "t_t") — the asymmetry tanked the similarity score. Now '/ \ : _'
all map to spaces before the strip, so "/ T:T" and "_ T_T" both normalize to "t t".
Verified on the real library: similarity for the Sawano pair 0.927 -> 1.000; only
348/40786 strings (0.85%, all containing those separators) change; worst-case
joined-variant match (e.g. "12:05" vs "1205") stays 0.889, well above the 0.70
title threshold — no match regressions. Fixes the matching half of #851.
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