BCT Letter 86: The Cabibbo Angle from BCT Void Geometry — sin (θC) = 2rₜet (1+3α0/8) at Sub-0. 1%

Letter 86 of the BCT Superfluid Lattice Model programme. The Cabibbo angle — the fundamental parameter governing quark flavour mixing, unmeasured from first principles since Cabibbo (1963) — is derived from BCT void geometry with zero free parameters. The tree-level result sin (θC) = 2rₜet = (√6−2) /2 gives −0. 264% error. The NLO Josephson correction (1+3α0/8), where 3 = D4 triality sectors and 8 = D4 fundamental representation dimension, gives sin (θC) = 0. 225369 vs PDG 0. 22534, error +0. 013%. Sub-0. 1%. The Cabibbo angle is the diameter of the BCT tetrahedral void. The complete two-generation CKM block Vᵤd, Vᵤs, Vcd, Vcs follows from unitarity with no additional parameters. Prediction #123.