Testing: property-based tests (FsCheck.Xunit) for domain invariants

Context: No property-based testing in the API.
Do: Add FsCheck v3 + FsCheck.Xunit ([Property]). Cover domain invariants: overdue/DueDate rules, streak monotonicity, skip semantics, XP/gamification bounds, serialization round-trips. Pure domain logic only.
Gate: part of the unit suite (BLOCK).
Why: same as thomasluizon/orbit-ui-mobile#418.