Instruction file imported from vaags/regneflyt (.github/instructions/domain.instructions.md). Copyright stays with the author.

Regneflyt Domain Boundary

• Organize code by business responsibility: arithmetic, skill progression, puzzle generation, and quiz behavior.
• Keep domain modules independent of Svelte, SvelteKit, Paraglide, browser globals, stores, contexts, routes, components, and Vite environment flags.
• Import domain dependencies from canonical #lib/domain/** paths. Do not import compatibility files under src/lib/helpers, src/lib/models, or src/lib/constants.
• Keep calculation APIs direct and deterministic. Inject time or randomness only where behavior varies.
• Preserve RNG call order when moving or changing puzzle-generation logic.
• Avoid dependency containers, service locators, broad barrels, and runtime registries.
• Keep user-facing labels and formatting in integration or UI modules.
• Run npm run check:domain, targeted domain tests, npm run verify, and npm run test:bundle after non-trivial domain changes.