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RFC 5545 §3.1 content line unfolding.
Physical lines longer than 75 octets are folded by inserting a CRLF followed by a single SPACE or HTAB. The reader must reverse this transformation before per-line parsing — the leading folding-marker whitespace is part of the fold, not the value.
Also handles a leading UTF-8 BOM (U+FEFF) tolerantly: many tools
(notably Outlook) emit one, and the rest of the parser assumes the
input starts at the first wire character.