Format size benchmarks: check that every arm stores the same signals (real-valued VCD vars are often dropped)

When comparing waveform or trace formats on file size, a converter or writer that silently skips some signal kinds produces an unfair size win. Real-valued VCD variables ($var real) are the common case: some converters and writers drop them or don't support them, while FST keeps them.

Checks that caught it: - Per workload, count real-valued signals and their change events in the source VCD, and record their share of all changes and bytes. - Compare signal by signal across formats, not as a set of value streams. Two signals with swapped streams pass a set comparison. Normalise names first (for example, strip a trailing [msb:lsb] suffix), and fail the workload if normalising makes two names collide. - Emit a mapping file per workload: for each signal, its name, width, change count and checksum in every format. Later per-signal read benchmarks can then be matched exactly, and any missing signals are listed rather than hidden. - If one format lacks a signal kind, build the competitor's file from a copy of the source with those signals stripped, using the same settings, and compare size against that. Otherwise label the cell 'excludes N signals (X% of changes); not like-for-like' and claim no size win. - Log the missing signal kind as a feature gap, not a footnote.