nx_regalloc_soundness_test.nx

nx_regalloc_soundness_test.nx -- BITS-UP soundness proof for the allocator running on compute_liveness' (correct) intervals. Builds a loop where K values are all live across the body (defined in entry, used in the header each iteration -> live across the back-edge), runs the FULL allocator flow WITH compute_liveness enabled, then checks the interference invariant: for every pair of values that are BOTH register-homed and whose (post-liveness) intervals OVERLAP, their registers MUST differ. A violation = linear-scan handed two simultaneously-live values the same register = a real miscompile source. Zero blast radius (executes the allocator, not the built IR). exit 0 = sound. 50 = an interference violation (prints the pair). other = setup divergence. license_tier: ORIGINAL