Scalar-Derived Gauge Closure of the QDL–SO10–1 Benchmark: Explicit Threshold Blocks, Dynkin-Index Accounting, and the v0.3 Closure Test

The QDL-SO10-1 grand-unification sequence defines a closure-first SO(10)-compatible benchmark within the Quantized Dimensional Ledger (QDL) program. Papers #1-#3 established the initial benchmark trilogy: a fixed SO(10)-compatible benchmark, its low-energy phenomenology, and a stress-test / robustness audit. Paper #5 began the executable hardening sequence by revising the gauge-closure target to inverse alpha_U = 44.501718, corresponding to alpha_U = 0.022471, and by converting the remaining Pati-Salam channel splittings into calibrated high-scale threshold targets. This paper supplies the v0.3 scalar-threshold hardening layer. The derivation is explicitly block-level: it proves that the calibrated v0.2 offset vector lies in a declared Dynkin-index threshold span, but it does not yet constitute a full component-by-component SO(10) scalar census. Three aggregate threshold blocks are introduced with traceless Dynkin-index splitting vectors d4 = (12, -6, -6), dL = (-6, 12, -6), and dR = (-6, -6, 12), equivalently corresponding to aggregate Dynkin-index blocks S4 = (18, 0, 0), SL = (0, 18, 0), and SR = (0, 0, 18) after common-shift subtraction. Using the high-scale threshold formula lambda_i = -(1 / 12 pi) times the sum over threshold blocks of S_i times ln(M_a / M_U), with M_U = 1.00 x 10^16 GeV, the v0.3 solution is ln r4 = 2.689679, ln rL = -1.989712, and ln rR = -0.699967. The corresponding threshold masses are M4 = 1.472695 x 10^17 GeV, ML = 1.367348 x 10^15 GeV, and MR = 4.966016 x 10^15 GeV. The derived threshold vector is (-1.284227, 0.950018, 0.334210), with RMS threshold residual below 10^-12 at displayed precision. The result advances QDL-SO10-1 from calibrated executable gauge closure to block-level scalar-derived gauge closure. It is a proof-of-principle scalar-threshold derivation layer, not the final microscopic scalar-sector completion. The next burden is a full component-level SO(10) to Pati-Salam to Standard Model scalar census and QDL admissibility verification.