byteSize

Total encoded length of this value in bytes, including its header.

For a container this reads the last field offset rather than walking children, so it is O(1) whatever the value holds.

The extent is checked against the segment before it is returned. This is what a caller uses to step from one stored value to the next, so a length that runs past the end of the data must be reported as unreadable here rather than turning into a wild offset one step later.