MRET v4.2 presents a one-parameter, horizon-coupled account of late-time cosmic acceleration with no new exotic fluids and no modified gravity. The dark sector is reframed as one spacetime fabric in two behaviors: a clumped, dust-like mode (EDR) that reproduces dark-matter phenomenology, and a smooth, horizon-coupled reservoir (HCA) that mimics dark energy, but as a symptom, not a fuel. When structure forms, especially as black holes grow, the horizon ledger reclassifies a tiny fraction of geometric energy from EDR to HCA. This single-dial exchange (Γ₀) is timed by a fixed, unit-area kernel tied to the black-hole accretion history, preserving early-universe successes while producing gentle, falsifiable late-time signatures.MRET predicts a +2–5 mmag supernova residual peaking near z≈0.5, Λ-like growth with a small, localized damping term, and a percent-level H₀ uplift with the correct sign (local > early). A null result cleanly tightens an upper bound on Γ₀ (AIC +2 penalty). This record includes the full preprint (technical) and a lay explainer. As an independent researcher, I welcome collaboration, critique, and opportunities to scale this work into full-time development.