nx_editor_canvas_gate.nx

nx_editor_canvas_gate.nx -- the editor engine: snap/align/distribute/nudge, the ORIGINAL one-deep snapshot undo, and the OPERATION-STACK adoption that gives the canvas real multi-level undo, redo, bit-exact replay and re-evaluation at depth. PART A IS A REGRESSION GUARD AND IS DELIBERATELY UNCHANGED. T1..T6 are the six assertions this gate already made, on the same fixtures with the same expected numbers, exercising ec_snap_all / ec_align / ec_distribute_h / ec_nudge / ec_save / ec_undo through the ORIGINAL direct-call path. They are also the POSITIVE CONTROL for part B: if adopting the stack had broken the engine underneath, or if the new guards refused work they should allow, these six would go red. PART B PROVES THE ADOPTION, and its anti-vacuity tooth is T11. A one-deep snapshot passes a single undo -- T9 alone cannot tell the two designs apart. Only undo-undo-undo followed by redo-redo-redo separates a log from a snapshot, because the snapshot holds exactly one state and has nothing to return to on the second call. T17 is the second anti-vacuity tooth: it proves the canvas the caller renders IS the stack's own memory rather than a copy kept in step by hand. 100% sovereign. No hardware writes (Rule 26). expect_exit: 0