nx_compute_graph_test.nx

nx_compute_graph_test.nx -- typed DAG end-to-end shape verification. Constructs a small computation graph that resembles a tiny ML forward pass: two inputs feed a matmul; the matmul feeds a ReLU; the ReLU is the output. Validates that: * structural validation accepts the well-formed graph * toposort emits a valid linearisation * cycle detection refuses a graph with a back-edge * dangling input detection refuses a graph with unwired slot * bad-edge detection refuses an edge to a non-existent node * content hashes are stable across builds with identical shape