X72-50: Invisible-Manifold Source-Lock Characteristic Geometry

Building on Round 49's exclusion of the golden manifold, this round stops testing family by family and instead builds a second linear filter — the linearization of source cancellation — alongside the existing state normal operator, giving a characteristic classification of isolated Fourier perturbations for which both state and source cancellation hold simultaneously. Proves that Round 48's entire horizontal hidden plane, once passed through the source filter, collapses to two characteristic circles; the non-horizontal isolated hidden surfaces all vanish except at the Beltrami resonance point; and the two surviving source-hidden circles remain non-Beltrami, with nonzero quadratic lift. Route halts at STOP-C54 (second-filter characteristic / nonlinear-invisible-curve gap): first-order state-plus-source hiddenness still does not amount to the genuine persistence of a nonlinear invisible manifold, handed to the next round for a second-order manifold correction; this round also includes a numerical-verification script.