Function reassemble_group 

pub fn reassemble_group(part_lines: &[String]) -> Option<String>

Reassemble one complete group of EVENT-PART lines back into the original <EVENT attrs>body</EVENT> line, byte-for-byte. Returns None (silent orphan drop) when the group is incomplete or malformed:

• no head part (seq="1/M") present — earlier parts lost across a session boundary,
• missing any seq in 1..=M,
• inconsistent denominators,
• more than one distinct id (use reassemble_stream for mixed streams).

Parts may arrive in any order; they are sorted by seq numerator here.