raposza/tools: platform/raposza-sandbox/README.md

<!-- Copyright (c) 2026 bentzn --> <!-- SPDX-License-Identifier: Apache-2.0 --> # raposza-sandbox What a sandbox IS: the STACK. Two of them - a 3.x `SandboxStack` and a 2.x `Sandbox2xStack` - plus the sandbox spec, the launcher choice, and the process that runs Canton's own `sandbox` subcommand. Eve