RCO-A2A - GSC rail

The GSC rail: records GSC issues, signed with GSC keys.

Counts at birth - record-holding: 1000 · prefab: 1000. Machine truth: index.json.

Slot agents: /<slot>/.well-known/agent-card.json and /<slot>/a2a.

Records: /records/<record_id>.json · ledger receipts: /ledger/<record_id>.json.