Mazocarta is a seeded procedural tactical deckbuilder implemented in Rust, compiled to WebAssembly for browser play, and executable natively for simulation. Its primary technical contribution is not the invention of a new deckbuilding genre, but the construction of an instrumented game-development reference artifact: the same rules engine supports interactive play, native command-line simulation, automated end-to-end tests, save/load fixtures, and local-area multiplayer. This paper describes Mazocarta's architecture, deterministic run model, reproducible balance probes, and QR-mediated WebRTC pairing for local multiplayer. An evaluation snapshot over 1,000 deterministic seeds shows that the simulation pipeline can produce reproducible development signals. In the evaluated configuration, single-player and two-player autoplay win rates were 36.1% and 34.9% over 1,000 deterministic seeds, respectively. These rates are not presented as final player-facing balance metrics, but as repeatable probes for future balance shifts and regressions. Mazocarta is positioned as a playable open-source reference artifact for instrumented game development: deterministic regression checks, automated playtesting workflows, balance probes for game mechanics, and browser-native local multiplayer all exercise one shared production rules core.