fix(redshift): reconcile must not return Found on a node another lineage claimed #711

Problem

The Redshift ReconcilePurchase (#708) treats every owned node in the attempt's window as a candidate. #710 makes the duplicate guard skip nodes that another purchase lineage recorded as bought (common.ClaimReader). Reconcile doesn't consult that read yet.

Two lineages can buy the same offering and count in overlapping windows. If our attempt never landed, reconcile can return Found on the other lineage's node. That hides a failed purchase. EC2 avoids this through its idempotency tag rule.

Proposal

Whichever of #708 and #710 merges second routes Redshift reconcile candidates through the same claim read. A node another token recorded as purchased counts as an uncertain candidate, so the result is Unknown, never Found. A read error is Unknown with its own Reason.

Add a test: one window node, claimed by another token, gives Unknown. Do this before the platform calls ReconcilePurchase for Redshift.