We present a Jordan-frame scalar–tensor theory in which a conformal scalar φ dynamically rescales the gravitational coupling and geometrically absorbs the vacuum energy. The observed cosmological constant emerges from the slow late-time evolution of φ, tied to the cosmological horizon, while the quantum vacuum contribution is suppressed as ρ_eff = ρ_vac / φ² — encoding the 120-order-of-magnitude discrepancy in the dynamical history of φ rather than fine-tuning. General Relativity is recovered locally, and ΛCDM arises as a degenerate fixed point when φ̇ → 0. At the perturbative level, a φ-dependent dark matter mass induces frame-invariant non-adiabaticity, leading to a redshift-dependent effective gravitational strength μ(z) = φ₀ / φ(z) > 1 at high redshift. This preferentially boosts non-linear halo collapse, accounting for massive early galaxies seen by JWST, while preserving near-ΛCDM linear growth and lensing potentials (Σ(z) ≈ 1), consistent with RSD and weak-lensing constraints. A distinctive falsifiable prediction is a redshift-dependent violation of the GR relation μ = Σ, yielding a suppressed E_G statistic with E_G(z ≈ 3) ≈ 0.302 (versus ≈ 0.321 in GR/ΛCDM). The theory is free of ghosts and instabilities, and offers testable predictions for Euclid, DESI, JWST and next-generation CMB experiments.