GetUserBalance

A live ledger balance for a user. amount and pending are u128 minor-unit decimal STRINGS (proto3 has no u128; a string is lossless and matches proto-loader's longs: String). An investor account is credit-normal, so amount = posted credits − posted debits is non-negative by invariant. authoritative is true when read directly from TigerBeetle; as_of is unix seconds. The authoritative/as_of pair is the forward-compat seam for a future read projection — a projection may later return authoritative=false rows with no breaking proto change. (Named for the users surface; the company-money BalanceService owns its own balance messages.)