Strain Coupling, Mode Structure, and Experimental Predictions from the Quaternionic Vacuum Condensate

The vacuum mechanics framework derives 61 Standard Model observables from a single parameter ν = 0.4555. We derive the universal strain coupling g = MK√π from first principles, showing the electron mass follows with zero free parameters in the lepton sector, reducing Standard Model parameters from 26 to approximately 6. The quaternionic condensate has exactly seven independent fluctuation modes (four internal, three spatial), each with distinct coupling to matter. Analysis of seven experimental approaches using Z₂ parity constraints finds only acoustic strain coupling viable, with all electromagnetic experiments failing by 7 to 24 orders of magnitude. We predict a specific spectral signature for the acoustic experiment comprising two independent observables (mass shift and birefringence), including a geometric null at the fourth harmonic. Testing the framework's prediction that the fine-structure constant varies with gravitational potential using 295 quasars and 43,506 galaxies, we find that systematic investigation reduces an initial 3.1σ signal to the known Webb dipole — an honest null for the gravitational coupling hypothesis.