Function compute_content_hash 

pub fn compute_content_hash(
    source: &[u8],
    start: usize,
    end: usize,
) -> Option<String>

Compute a normalized content hash for duplicate detection. Returns None if the content is too small (< MIN_HASH_SIZE bytes).

Normalization:

• Strips leading/trailing whitespace
• Collapses all internal whitespace to single spaces
• Uses blake3 for fast, high-quality hashing
• Returns first 16 hex chars (64 bits) - sufficient for grouping