Move cost calculation to backend

[corbt]

Summary

• Introduce centralized cost utilities and a model-level cost calculator (default no-op)
• Compute training costs in TinkerNative backend and remove trainer-side cost inference
• Aggregate arbitrary costs_* components; reject manual costs_total
• Log costs at current step with an instance-scoped lock
• Update binary prefix pipeline to use the model cost calculator

Testing

• pre-commit hooks via commit (ruff, ruff-format, ty, uv.lock sync)