Verdi fsdbdebug -top_n_var 0 lists stored (primary) FSDB vars by idcode

To tell dumped (primary) FSDB signals from ones the reader rebuilds on load (secondary: port aliases, combinational wires), with no public FsdbReader API for it: run $VERDI_HOME/bin/fsdbdebug -top_n_var 0 file.fsdb. It prints only stored vars, one per line, as <idcode> /hier/path <vcCount>(<pct>%). The output goes to stderr after a banner, so keep only lines that start with a digit. The idcodes are the same var idcodes FsdbReader uses, and the change counts give exact size and cost estimates before loading. Cost: 1.7 GB file 4.4 s / 0.5 GB RSS; 10 GB file 27 s / 2.5 GB RSS, so run it once per file and cache the result. Avoid -with_computed_signal on big files: it loads every secondary and went past 6 GB. -dump_dealias_tbl is not an alias-to-primary map; most of its masters are themselves secondary.