Function protected_segments_present 

pub fn protected_segments_present(segments: &[Vec<u8>], output: &[u8]) -> bool

Invariant 3: every protected-content segment (system turns, latest user message, diff headers) must still be present, byte-for-byte, somewhere in the output. Segments are checked individually (see budget::protected_segments) because concatenating them into one blob would rarely be contiguous in the real document.