Paper CCXLVIII: Positronium Diffraction (Nagata 2026) as the Time-Reverse of Paper XL Pair-Production Lock

Nagata et al. (Nat. Commun. s41467-025-67920-0, 2026) report positronium (Ps) diffraction with peak at λ_Ps = h/(p_e- + p_e+) = λ_e/2 = 8.1 mm, not the 16.2 mm independent-particle prediction. We identify this operationally with the time-reverse of Paper XL's pair-production lock: the measured single-amplitude de Broglie wavelength is the operational definition of the intermediate state (coherent brane amplitude with both H+ and H- contributions) that Paper XL's γγ → e+e- lock mechanism requires but did not directly measure. Nagata's measurement is in the time-reverse direction (Ps in flight, pre-annihilation), promoting Paper XL's intermediate state from unobserved derivation step to measured fact. Three falsifiable predictions: (P1) coherence-breaking external field F_c ≈ 1.6 × 10^10 V/m (= F_Stark,Ps × sqrt(1/14)) at which the 16.2 mm independent-particle peak appears in a Nagata-style apparatus; (P2) ε_Ps/ε_e = 1/14 = 0.0714 from G2 root count, matching observed 0.067 at 7% (vs Paper CXCIV's f_bulk = 0.0369 off by factor 1.8); (P3) Schawlow-Townes-narrowed linewidth in Ps BEC stimulated 511 keV emission. Clinical link: J-PET positronium lifetime imaging predicts τ_tumor/τ_healthy ≈ (F_eff,healthy/F_eff,tumor)² with squared F-dependence from ε_H,min-threshold structure; reported PLI ratios 0.85-0.95 → 5-10% effective-field modulation. Companion to Paper XL (pair production lock) and Paper CXCIV (structural Ps = H+ ⊕ H-). Part of the One-Octonion Brane-Bulk Framework series. Anchor DOI: 10.5281/zenodo.19120873. Community: one-octonion-brane-bulk. Author: Bharathi Dasan Jagadeesan, M.D., University of Minnesota. ORCID: 0000-0002-1143-941X.