BCT Letter 86: The Cabibbo Angle from BCT Void Geometry — sin(θC) = 2r_tet(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) = 2r_tet = (√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 {V_ud, V_us, V_cd, V_cs} follows from unitarity with no additional parameters. Prediction #123.