Standard physics treats the kinematic null boundary in Special Relativity (v → c) and the gravitational event horizon in General Relativity (r → rs) as two distinct asymptotic limits. This paper proposes a unified framework that reinterprets both limits as manifestations of a universal null topological seam (ds2 → 0). We posit that elementary particles do not terminate at these boundaries, but rather undergo a discrete topological transformation of their proper causal flow. By elevating the geometry to the cotangent bundle, we show that crossing this null boundary triggers a symplectic involution (Ω → −Ω). This rigorously reverses the canonical Hamiltonian vector flow, acting as a geometric CPT inversion while leaving the background metric (gμν) invariant. Applied to flat spacetime, this provides a kinematic engine for the Feynman-Stueckelberg matter-antimatter annihilation vertex. Applied to gravitational collapse, it yields a "Black Mirror" topology where the interior singularity is replaced by an effectively repulsive transition into a conjugate anti-universe. By unifying these boundaries, we establish a continuous bipartite phase space where a single oscillating worldline generates infinite generations of matter and antimatter.