feat(citation): the Sigla facade, total by construction

format never raises and returns an unparseable citation unchanged, so a
gap degrades to today's behaviour rather than to a crash. The coverage
test asserts separately that no shipped citation takes that path.

verbatim is what --raw uses: an identity name table would still reformat
punctuation and renumber, which would break byte-exact diffing against
lectio.

The existing citation-coverage walk (1970-2070) now also drives a
round-trip check in the same pass: parse -> render -> parse must reach
the same structure, using Book.default_spelling for names (Book.to_string
returns the internal id, which is not a registered token and cannot be
read back). Proved with a mutation: changing the default style's
part_sep to a separator the parser does not accept reddened the
round-trip case on 31 multi-part citations and nothing else; reverted.

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