Cost accounting

CostRecord keeps estimates and charges apart:

kind	Meaning
estimate	from a provider PriceQuote at provisioning time
provider_usage / provider_charge	reserved for real adapters that report usage/billing
internal	GPUlayer-calculated accounting

Every record has amount, currency, unit (per_hour or total), source and recorded_at. A missing price is recorded as unknown, and when max_hourly_cost is set, an offer without a price is rejected. Today only FakeProvider produces estimates, and they are labelled as simulated.

Where cost data comes from

Provider adapters attach a PriceQuote to each offer when the provider publishes a price. When GPUlayer provisions an instance for a job, it records an estimate for that job. Estimates appear in JobResult.costs (gpulayer job result <id>).

No real provider adapter exists yet, so in practice only the simulated FakeProvider produces estimates. Workers that joined with gpulayer join have no price attached.