Select scoped memory records

Implement select_memory(records, user, now, limit). Each record has id, scope (* or a user), at, expires (integer or None), tier, and key. Exclude other users, future records, and records whose exclusive expiry has arrived. For each (tier,key), prefer the user's scope over global scope, then the latest timestamp, then the lexicographically greatest identifier. Sort winners by descending timestamp and ascending identifier; return at most limit. Raise ValueError for a negative limit. Inputs otherwise satisfy this schema. This is a memory selection algorithm, not an identity provider.