feat: header content_type (byte 64) + i64 acquisition_ns timestamp; bump to 1.2.0
Surface the on-disk content_type descriptor and the precise i64-nanosecond
acquisition timestamp on Header (parse_header / build_header_bytes round-trip
them), with a content_type_name tag. Backward-compatible: legacy captures
decode content_type as Undefined (0) and keep their ASCII timestamp.