feat(lang): read [bible] and [sigla] sections

Lang reads a hardcoded list of section names and ignores anything else
silently, so without this a [bible] section would appear to work and do
nothing -- confirmed directly with a scratch executable: before this
change, of_string on text containing a [bible] section parsed without
error and Lang.keys came back empty, no trace of the section anywhere.

Lang.bible keeps the total-lookup contract every other lookup in this
module has: a miss returns the key itself, never the empty string.
Lang.sigla_fields returns the [sigla] section's raw fields, values
trimmed but still quoted, for Render.style_of_fields to unquote.

[bible] joins the keys reference set so lang --check reports missing
book names; [sigla] deliberately does not, being settings with working
defaults rather than translatable names -- adding it would make --check
demand five settings from every language file.

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