v10.19.0 — D-algebraicity strategy redirect (P4)

D-algebraicity of U remains OPEN. This release documents why the natural strategy (extending the pole-density argument that proves non-D-finiteness to the nonlinear case) does not work: algebraic ODEs can have movable (Painleve-type) singularities dense on a curve, so pole density alone is not an obstruction the way it is for linear ODEs. The correct theorem family is identified (Malmquist/Steinmetz/Bank-Kaufman growth theorems), with the disc-analog version flagged as unconfirmed in citable form. An initial draft's flagship counterexample was found broken on adversarial review and is disclosed and retracted in the note; no false claim survives. No new theorem is claimed.