Function find_quick_boundary 

pub fn find_quick_boundary(
    buf: &[u8],
    start_bit: u64,
    max_blocksize: u32,
) -> Option<BlockBoundary>

Find a quick-verified block boundary starting from start_bit.

Forward-scans for the 48-bit magic, then checks 73 bits of block header structure to reject false positives. No full decode needed.