The generated models follow the spec: TokenCounts, Cost, Usage,
ModelUsage, UsageModelRef, UsageStageRef, Speed, RunUsage,
RunUsageStage, RunUsageTotals, UsageByModel, AggregateUsage, and
AggregateUsageTotals replace the billing models, and UsageApi replaces
BillingApi. The stale billing model files are deleted.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Replace the overlapping usage and cost model with canonical billing
primitives centered on ModelRef, ModelHandle, TokenCounts, and
BilledModelUsage. This also renames the public API and web surface from
usage to billing, removes compatibility aliases, and normalizes provider
usage adapters onto the shared billing vocabulary.
