VCS: vpi_control(vpiStop) needs -debug_access+line, or the stop never lands

VCS W-2024.09. A VPI library that parks the simulation for UCLI (vpi_control(vpiStop, 0), then UCLI checkpoint -add under -ucli -ucli2Proc) only gets the prompt back if the build has line debug capability. Measured on a small fixture, with the same library and test, varying only the elaboration flags: - -debug_access+r+w+f+cbk, and the same plus +thread, +drivers or +pp: the stop never takes effect. The simulation runs to $finish, and the checkpoint request never gets a prompt. - Add +line, or use +designer, +cbkd or +all (all include line): the stop takes effect at the next HDL activity, and the checkpoint works.

Plain UCLI is different. run 50; checkpoint -add; run 40; checkpoint -join 1 works even at bare -debug_access, so only the VPI-requested stop needs +line. Built with only +vpi and no -debug_access, UCLI run itself fails with Error-[ACC-STR] Design debug db not found.

+line has a cost. On a CVA6 testbench, 50K cycles, it was about 1.1 s of a 6.5 s run, even scoped to the TB top with -debug_region=1,<tb>. A SIGINT to a plain -ucli simv also returns the prompt without +line. Under -ucli2Proc, though, I couldn't get the prompt back that way within 4 s (inconclusive).