Theorem: H_0 d_L - c z = 0 (standard-siren Hubble identity at low z)

Theorem (standard-siren-Hubble canonical): H_0 = c z/d_L cleared gives H_0 d_L = c z, residual H_0 d_L - c z = 0 identically. Canonical sympy pins: c, z, d_L = sp.symbols('c z d_L', positive=True); H_0 = c*z/d_L; residual = sp.simplify(H_0…