Terminal Residue Sieves in the Single-Defect Stratum of a Width-Two Collatz Survivor System

We study a finite terminal residue sieve arising from the single-defect stratum of a width-two accelerated odd Collatz survivor system. Using the exact top-bit realization criterion for class-7 defects together with the class-3 mod-6 realization law, we define the residue classes not certified by the first R terminal witnesses. We prove that, over the natural common period, the exceptional set has exact cardinality 2R for every R >= 2. The proof uses half-period complementarity of the terminal characters, uniqueness of a zero-witness 2-adic branch, and a two-lift recurrence. The result is finite-depth and does not establish diagonal avoidance, the general width-two dominance conjecture, or the classical 3x+1 conjecture.