Scripting fsdbreport/fsdbdebug: xtag pairs, -bt units, stray log dirs

When scripting Verdi W-2024.09's FSDB utilities to find x/z intervals: (1) fsdbdebug -info prints maximum xtag : (15 424465560) or (64848975000ps); the pair is the high and low 32-bit words, so dumps past ~4.29 us at 1 ps give a wrong end if you parse the second number; parse the or (Nps) form. (2) fsdbreport -bt/-et default to ns; write -bt 123ps. -csv -of h -s /tb/u/sig -o r.csv gives one row per change with full-width values (the table format truncates wide buses with *); x/z show as letters, so 'x' in v or 'z' in v finds x spans. Memory elements are addressable as /tb/u_ram/mem[3328] when dumped with +all. (3) Each tool drops a fsdbreportLog/fsdbdebugLog dir in the cwd (your home if run from there), so run them from a scratch dir with an absolute FSDB path. (4) After module load of the Synopsys stack, python3 on PATH can be a bundled 3.6 (no subprocess capture_output); call /usr/bin/python3 explicitly.