Record format

Each record is a JSON object serialized canonically (keys sorted, no whitespace):

• v, record_id, machine_id, site_id, hour_start (ISO UTC)
• gpu_count, samples, gpu_seconds_raw, gpu_seconds_billed
• energy_wh, rate_micro_per_gpu_hour, billed_micro (strings for integers above 2^53)
• prev_hash — the previous record hash for the same machine
• signer — hex Ed25519 public key

record_hash = SHA-256(canonical). signature = Ed25519(record_hash).