* fix(extractor): don't flag gid Differences names covered by ToUnicode
Pages were marked as having unresolvable gid-encoded fonts whenever any
font's /Differences array used gidNNNN glyph names, and when every page
carried such a font the whole document's markdown was suppressed.
LibreOffice exports do exactly this: subset fonts get /gidNNNN names in
Differences alongside a complete ToUnicode CMap that decodes them, so
ordinary text documents lost their entire markdown output even though
extraction decoded every glyph.
Track the character codes behind the gid names and only flag the font
when its ToUnicode CMap addresses none of them. Partially mapped codes
stay unflagged: an emoji ZWJ sequence maps whole on its first code, and
the remaining component-glyph codes are subset leftovers, not damage.
Fonts without ToUnicode, or whose CMap ignores the gid codes, are
flagged as before, and the downstream garbage/encoding checks still
catch partial breakage.
* fix(extractor): require a usable ToUnicode mapping to clear the gid flag
A mapping to U+FFFD (or an empty string) is rejected by extraction as
an invalid CMap result, so it must not count as decodable when deciding
whether gid-named Differences codes are resolvable.
