A blank first line caused the format detection regex chain to fall
through to the error exit. Read lines in a loop and skip empty ones
before matching. Add tests for blank lines with LF, CRLF, whitespace,
and BOM variations.
Use utf-8-sig encoding to natively strip BOM and remove null bytes from
UTF-16 artifacts so the bare hash regex matches correctly. Replace the
unhelpful "unknown format" error with a diagnostic message showing the
actual first-line content and expected formats.