Large-scale inverter-based generation presents a fundamental coordination challenge during planned or emergency islanding events: how can hundreds of geographically dispersed solar inverters simultaneously establish a stable island without real-time communication? Three NERC- documented solar PV coordination failures motivate this work: Blue Cut Fire 2016 (1,178 MW lost, NERC/WECC 2017), Odessa I 2021 (over 1,100 MW, NERC/Texas RE 2021), and Odessa II 2022 (1,711 MW — largest documented solar IBR tripping event in US history, NERC/ERCOT 2023). In all three events the root cause was identical: dispersed inverters responding individually with no coordinated response. This paper proposes an architecture in which all inverters within a designated island cluster are pre-programmed with a shared island-mode P–f droop law, defined by a 45 Hz nominal frequency and droop coefficient kp: f = 45 − kp(P − P*). Upon islanding detection — achieved passively through rate-of-change-of-frequency exceeding a 5.83 Hz/s threshold derived from verified South Australia 2016 event parameters (M = 3,000 MW·s, ΔP = 700 MW) — each inverter autonomously adopts this droop law, eliminating the need for inverter- to-inverter coordination or communication. VFD centrifugal loads reduce island power consumption by 27.1% as frequency drops to 45 Hz (affinity law: P ∝ (f/f0)³; IEEE Task Force 1993). A hard physical ceiling is established: for generation losses exceeding 27.1% of island capacity, battery fast-frequency response (FFR) is mandatory regardless of load mix. For the SA 2016-scale event (38.9% generation loss), battery FFR requirements range from 118 MW to 389 MW per GW of island depending on the VFD load fraction (α). The architecture is positioned within IEEE Std 1547-2018 Clause 8.2 intentional islanding provisions. Inverter protection reconfiguration requirements for continuous 45 Hz operation are specified. A structured prior-art search across IEEE Xplore, USPTO, and Chinese patent databases finds no prior work using fixed off-nominal frequency as the GW-scale island coordination primitive. Three simulation requirements are identified before journal submission: cloud transient stability, impedance- dependent droop coefficient optimisation, and passive resynchronisation phase-angle verification.