fsdb2vcd declares packed-struct members but emits no values for them

When comparing an FSDB dump against other formats by converting it with fsdb2vcd, packed-struct members can show up in the VCD header with no value changes. The FSDB's own reader sees the struct as one vector, with data. Counting 'signals' or changes from fsdb2vcd output then inflates the declared-name count and undercounts changes. VCD/FST copies made through fsdb2vcd (e.g. fsdb2vcd then vcd2fst) also lack packed-struct values. For parity checks, compare structs at vector level against the FSDB reader directly, and derive members by slicing each vector by the declared member ranges. Also check any VCD exporter's $timescale label against its actual tick unit; one exporter wrote 1ps while emitting ns ticks.