BPB/MBE HR Rational Branch: Exact a0 = cH0/5 Priority Technical Note

This priority technical note records an exact rational identity in the BPB/MBE biface-genesis branch of ghost-free Hassan–Rosen bigravity. For the binomial HR cosmological potential VgF (y) =β0+3β1 y+3β2 y²+β3 y³ with β0=459/500, β1=-27/100, β2=3/5, β3=-1 and yₜ=3/10, the branch gives VgF (yₜ) =81/100, ΔΩ=81/500, Ωₚerm=β0+β1=81/125, sHR=Ωₚerm/VgF (yₜ) =4/5, and therefore 1−sHR=1/5. The corresponding exact structural acceleration scale is a0HR=cH0/5. The note explicitly distinguishes this rational HR branch value from the source-locked HAL corpus value 1−sₛurv, 0=0. 2043075602291394. The package includes the technical note, LaTeX source, exact Python audit script, JSON/Markdown audit outputs, and hostile-review guardrails.