Since cd0b1a38, vk_pipeline.c lb_next calls lb_idle() and stops a boot builder at or above LB.bg_threads while the
builders run beside the game, so the check, which builds lb_next against a stand-in LB, failed to compile ("call to
undeclared function 'lb_idle'", "no member named 'bg_threads'"). The stand-in LB has bg_threads, and lb_idle copies
the real one with a test switch for bf_waiting. The concurrent runs now also cover two background workers and a start
that waits for the whole warm, and the explicit cases check that worker 2 of 2 stops beside the game and goes on in a
preparation. vk_pipeline.c is unchanged.
The check passes again with the ucrt64 clang.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
