nx_regalloc_liveness_diag.nx

nx_regalloc_liveness_diag.nx -- BITS-UP DIAGNOSTIC for the disabled compute_liveness. Builds the exact CFG shape build_intervals under- approximates: a value defined in the entry, USED in a loop HEADER, and NOT used after the loop. The back-edge (body -> header) makes the value live across the body, but its last LINEAR use is in the header (which precedes the body in block order), so build_intervals' def-to-last-use range stops too early -> linear-scan would think the register is free in the body -> clobber. compute_liveness must extend the interval across the body. This test EXECUTES the allocator (does not execute the built IR); it isolates the correctness question with ZERO blast radius to the self-hosting backend. Exit 0 = both facts confirmed (build_intervals too short; compute_liveness correct). Nonzero = where it diverged (see codes). license_tier: ORIGINAL