Horizon Response Principle (HRP) Addendum: The GW Comparator (Isaacson Flux Normalization)

This preprint is an HRP addendum note (“The GW Comparator”) in the Horizon Response Principle (HRP) suite. It is a triptych-style, constants-explicit normalization note that rewrites the Isaacson short-wave gravitational-wave (GW) energy flux in the HRP slot language k_SEG. This addendum is not a horizon sector: it is a non-horizon comparator channel, and no horizon-thermodynamic semantics are asserted. Scope:• 4D Einstein–Hilbert gravity only• Linearized gravitational waves in the Isaacson short-wave (high-frequency) regime on a smooth background• Plane-wave / wave-zone use in a locally inertial background patch (leading order in λ_GW/L)• Isaacson averaging ⟨·⟩ over scales λ_GW