A ledger shard per project, each with its own writer

No global single writer: the ledger is SHARDED per project (or a configurable shard group), each shard with its own database and its own off-main-thread writer, so projects write in parallel and capacity grows with them. The grouping default is UD-S6. Refines UR-01: one writer per shard is the single owner of that shard’s chain.