Reports a scalar ratio comparing the variance of the raw G_t entries (per element across t) to the variance of the canonical G_t entries (after per-period SVD). When the raw / canonical variance ratio is large (>= 5), the apparent G_t time-variation is dominated by rotation drift in U_t, V_t rather than real temporal change. The canonicalisation removes that rotation and the canonical entries should be the user-facing summary.
.rotation_drift_diagnostic(G_cube_raw, U_cube, V_cube)list with ratio (numeric), var_raw (RA x RB),
var_canonical (RA x RB), flag (logical: ratio >= 5)
RA x RB x T raw (FFBS-sampled) G cube
nA x RA x T raw U cube
nB x RB x T raw V cube