A hostel does not sell rooms. It sells beds, each on its own dates, its own rate and its own bill — and the people in them are strangers to each other.
The temptation is to build a second inventory model beside rooms. Vestibel does the opposite: a bed is a room row. That is not a shortcut, it is what lets beds inherit the pooled availability guard, the rate freezing, the nightly posting, the billing anchor and housekeeping without any of them learning a second grain.
It is also the shape the outside world already speaks. OTAs and channel managers express hostel inventory as a room type with a count of sellable units, which is exactly what this is, with no translation layer at the boundary.
What a bed cannot carry alone is which physical room it sits in and how many beds that room may hold. A small grouping entity carries only those two facts, and the bed-count limit is enforced under a row lock so two receptionists adding the last bed of a full dorm produce one bed and one refusal.