Cross-format waveform check: prove the 'extra' VCD/FST vars are aliases by their id codes

After matching per-signal hashes between a VCD-derived FST and another dump of the same simulation, the FST typically has thousands of vars that match no name on the other side. Don't wave them off as 'struct splits': classify them. (1) Vars whose path lies under a struct that the other format keeps whole are struct-field splits. (2) For the rest, read the VCD header ($var type width CODE name ...) and check whether each var's id code is shared with a var that was matched (or with a struct field): VCD/FST aliases (interface members seen through ports/modports, nested struct views) reuse the same code. If every leftover var shares a code, the converted file carries no data beyond the reference; any var with its own code is real extra data and needs explaining. In a 1.7 GB SoC dump this turned 716 'unexplained' vars into 220 aliases of matched signals + 496 aliases of struct fields, 0 with their own code.