Bulk resolve experiment assignments

Resolve experiment assignments for up to 250 units sharing the same unit type. Creates and persists assignments as needed (weighted hash or round-robin), identically to the single-unit endpoints. Designed for server-side SDKs (e.g. Roblox) that batch-assign players rather than polling per-user. Returns a map of distinctId → assignment array; IDs with no active assignments appear with an empty array. Fails the whole batch with 503 if any assignment or exposure record cannot be durably written.

Supply units with an optional per-unit context, plus an optional sharedContext carrying facts the whole batch shares — the unit's own values win per key. The older distinctIds array is still accepted but resolves without context, so targeted experiments will not match those units.