Resolution of the ATOMKI Nuclear Anomalies and the X17 Puzzle: Substrate Shear Cavity Resonances, Geometric Pair Locking, and the Elimination of the 17 MeV Protophobic Gauge Boson

High-precision electron-positron (e+e-) internal pair creation (IPC) experiments by the ATOMKI collaboration in Debrecen have revealed anomalous resonant peaks in the angular correlation spectra of de-exciting light nuclei: at θ ≈ 140° in the 18.15 MeV isovector transition of Beryllium-8 (>6.8σ) and at θ ≈ 115° in the 21.01 MeV isoscalar transition of Helium-4 (>7.0σ). Reconstructed invariant mass spectra across both systems consistently peak at m_X ≈ 16.7–17.0 MeV/c^2. In standard particle physics, accommodating this excess requires postulating an exotic "protophobic" gauge boson (X17) with fine-tuned couplings to evade stringent beam-dump constraints. Building upon our foundational series on the physical vacuum as an elastically pre-stressed continuum, we show that the 17 MeV anomaly is not a new fundamental particle, but an intrinsic acoustic shear cavity resonance of the vacuum substrate surrounding compact nuclear solitons. In our ontology, interacting nucleons generate an intense localized hydrostatic stress field (ΔP_nuc ~ 10^33 Pa) that stiffens the substrate shear modulus G_v(r). When an 18–21 MeV acoustic shear wavepacket nucleates an e+e- vortex pair within this confined envelope, the pair wavelength matches the spherical standing-wave mode of the near-field boundary: λ_pair ≈ 2π R_envelope ≈ 11.6 fm. This boundary condition enforces an invariant mass threshold m_res c^2 ≈ 16.85 ± 0.25 MeV, kinematically locking the angular opening to θ ≈ 140° for Beryllium-8 and θ ≈ 115° for Helium-4. The ATOMKI anomaly is thus proven to be a macroscopic manifestation of vacuum elastodynamics under Occam's razor, eliminating the need for hypothetical protophobic dark photons or fifth-force mediators.