Design the object model and public APIs for the booking core behind a boutique gym's class schedule, where members reserve spots in instructor-led sessions that each carry a fixed capacity.

Operating context. A studio publishes a weekly schedule of classes (spin, yoga, HIIT); each session has a start time, duration, instructor, room, and a hard capacity. A member books a spot, may cancel up to a cutoff before start, and rolls onto a waitlist when the session is full. When a confirmed member cancels, the earliest eligible waitlisted member is promoted automatically. A member may hold at most one active booking across any overlapping time window. Single studio, in-process service, one wall clock.

Out of scope. Payment capture and membership billing, the mobile / REST surface, calendar-sync exports (iCal), instructor payroll, and the persistence engine (assume in-memory for v1).

What to produce. The class hierarchy (entities, value objects, services), the public API each class exposes, and the state transitions for a booking (requested → confirmed / waitlisted → attended / cancelled / no-show). Be explicit about: how a full session waitlists rather than overbooks under concurrent booking attempts, how the promotion rule is pluggable (FIFO today, priority-tier tomorrow), and how the cancellation-cutoff policy is injected so it can be tested without waiting on a real clock.