Blueprint for Fault-Tolerant Trapped-Ion Quantum Computing: The Walking Cat Architecture All of that capital has been building toward one moment: a complete, buildable engineering specification for a fault-tolerant quantum computer that a team can actually execute against. That moment has arrived. The hardware capabilities this machine requires have already been experimentally demonstrated: high-fidelity two-qubit gates and reliable ion transport. IonQ is actively scaling physical qubit counts toward the thousands needed to run it. The Walking Cat Architecture is that specification. In 1945, John von Neumann's draft report on the EDVAC articulated the stored-program architecture that every classical computer since has been built on. It was not a theoretical proposal. It was a complete engineering specification. This paper is the quantum equivalent: the first end-to-end blueprint for a fault-tolerant quantum computer grounded in realistic engineering constraints, covering compiler, logical architecture, and micro-architecture for a machine capable of running millions of gates on hundreds of logical qubits. Just as von Neumann's architecture did not describe a single machine but the foundation of an era, this blueprint is designed to scale: it is the framework IonQ will build on through our 2030 objective of 2 million physical qubits and 80,000 logical qubits. The architecture is named for what it does at the physical level: cat states, walking. - "Cat" refers to cat states, a specific class of quantum resource state used to perform fault-tolerant logical measurements. A quantum computer cannot read a logical qubit directly without destroying the quantum state it is trying to protect. Cat states solve this problem: they act as probes, sent to interact with the logical qubits to check for errors, reporting what went wrong without collapsing the underlying computation. The concept traces back to Erwin Schrödinger's 1935 thought experiment, which birthed (and maybe killed) the famous cat. When