Density-Dependent Length Contraction: A Flat-Spacetime Model with Explicit Derivations V2

A flat-spacetime scalar model where gravity emerges as radial ruler contraction along the line-of-sight: s = max(1 - κ_eff ∫ ρ ds, 0.01), with κ_eff = 7.39 × 10^5 m² kg⁻¹ (Gaia DR3 calibrated) scaling as κ_eff ∝ 1/(1 + ρ/ρ_crit)^2, ρ_crit = 10⁻²⁴ kg m⁻³. Reproduces Milky Way rotation (220 km/s at 8 kpc → 195 km/s at 30 kpc) without dark matter, LIGO strain via density shear, redshift as λ_obs = λ_0 / s. Includes GPS clock match, proposed muonium lab test (Δf ≈ 10⁻¹⁰ Hz at 1 atm), and falsifiable predictions: JWST z>5 SN extra stretch, Swarm SAA cosmic-ray spike, NANOGrav nHz background from density variance. All open—no funding.