nx_print_runtime_zback_test.nx

nx_print_runtime_zback_test.nx -- Z-backward jump detection. Spaghetti progenitor: head crashes a tower mid-print, Klipper recovers by rehoming, Z snaps from current layer back to ~0. When the print resumes it does so off-position = spaghetti. Closed-form invariants: (a) Normal layer progression: no fire (b) Tiny Z-hop during travel (< tolerance): no fire (c) Z reverse > tolerance: NX_RT_ISSUE_Z_BACKWARD fires (d) Z_BACKWARD takes precedence over STUCK_LAYER (e) After backward event, monitor recovers (next normal event no fire) (f) Setter rejects negative tolerance (g) Tighter tolerance catches smaller backward jumps expect_exit: 0 license_tier: ORIGINAL