FSDB reader metadata can't prove a signal is stored, not computed

When benchmarking reads of stored FSDB signals, the documented FsdbReader metadata flags such as computed or middle are vendor-engine semantics, and all-false flags do not prove a signal is physically stored. A more conservative cohort is the set of idcodes that the vendor's fsdbdebug tool lists by default. In one dump it listed 38 of 47 selected signals; the 9 it omitted were packed-array or struct vectors that still had their own value-change blocks. Benchmark the vendor-listed cohort separately, and label the full selected set as storage not proven by API, rather than claiming everything is stored.