Function render_thread_enabled 

pub fn render_thread_enabled() -> bool

Whether a shell should run the render-thread split — the single switch every shell consults. true unless the NO_RENDER_THREAD_VAR kill switch is engaged (compile-time define or runtime env, any non-"0" value), mirroring crate::perf::enabled’s and crate::frame_gate’s option_env! + runtime-env parsing precedent.

Read once at shell startup: a shell that takes the split path spawns the render thread, a shell where this is false keeps the single-thread path verbatim.