Function regalloc_enabled 

pub fn regalloc_enabled() -> bool

Whether the contiguous register-allocating backend is selected. Default ON: it is bit-identical to the stencil tier (proven by the full corpus differential with the flag on) and 4-6x faster on the regions it supports, falling back to the per-piece tier on any unsupported op. LOGOS_REGALLOC=0 is the kill-switch. Read once.