Give every parser of the shared format layer a proper error enum
(thiserror): VersionParseError, ArchError, deps::ParseError (wrapping
the version and arch failures as #[source] so the context chain
survives), ControlParseError, ChecksumParseError, FilesParseError and
TestsControlParseError, each carrying its structured pieces (the
offending string, the stanza number, the field name) instead of a
pre-formatted String.
Display texts are byte-identical to the old format! strings, so
user-facing output and the message-asserting tests are unchanged; the
few tests asserting on error text now render the error first. Flow
boundaries that still take String (build::env, quirks, test::expand)
render with to_string() at the edge — typing those flows is a later
step. ControlInfo's FromStr impl follows parse_content's error type.
vhaudiquet
82a15c7426