nx_regalloc_interp.nx

nx_regalloc_interp.nx -- EXECUTION-equivalence verifier for register allocation. The static soundness checker (nx_regalloc_linscan: no two live vregs share a register) is necessary but the absorption gate wants more: PROVE that code run THROUGH the allocation (values living in physical registers + spill slots) computes the IDENTICAL result to the reference (one slot per value). Two interpreters, compared 1:1 over many inputs -- if they ever diverge, the allocation miscompiled (the g1 failure, caught before it can ship). The IR carries real opcodes so the interpretation is meaningful (it computes the race kernel's actual arithmetic). live-out values (loop-carried) are kept live to the block end so they remain readable -- exactly how a real backend treats them.