v10.20.0 — sharpened W_T non-D-finiteness gap (P5)

W_T's D-finiteness status remains OPEN (conj:WT-nonDfinite). This release sharpens the problem statement: non-D-finiteness follows immediately, by the same mechanism used for U and V, once a meromorphic continuation of F_T with infinitely many poles is exhibited. Three precise missing ingredients are named: no known continuation of F_T; the natural transfer variable is non-contracting, unlike the rational-base numeration systems the analogous toolkit was built for; no carry-catalytic transfer equation exists. No new theorem is claimed. Reviewed by Reviewer AI (CORRECT, no fixes required).