Get Zone Day Use Window

The dates a day-use zone's time-slot grid should offer.

Day use is seasonal and the season is published NOWHERE in the provider API — it is only visible in the availability grid, and it moves as the provider releases next season. So the grid asks for it per zone rather than assuming a fixed number of columns.

max_span_days rides along so the client can cap a SELECTION without capping the VIEW: the user browses the whole season but can only watch a window the platform's alert date-range cap allows.

closed_cells ({site_id: ["YYYY-MM-DD", ...]}) names the cells the grid must not let the user tap because their booking window has ALREADY shut. Day use is the one activity whose inventory dies during its own arrival day — each slot carries its own cutoff — so "not in the past" is not enough to know a cell is live. Computed here rather than shipped as raw cutoffs because deciding it needs the park's IANA timezone, and the app carries no tz database (Bruce is America/Toronto, Banff America/Edmonton). Read-time, never stored, exactly like a match's booking_closed.

Degrades to a bare cap (start/end null) whenever the window can't be read — an unreachable provider must not empty the grid.