CommitAsync now calls WriteAcquireHistory() between the two SaveChanges
calls: iterates _ops, skips SpendOps, writes one ViewerAcquireHistoryEntry
per GrantOp. Cascade rows get GrantSource.CardCosmeticCascade; wallet
currencies zero RewardDetailId; all rows in a single commit share one
DateTime.UtcNow timestamp. Closes _source plumbing from Task 5.
5 new tests added (46 total inventory, 0 failures).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>