Pure billing-attribution semantics for MCP 2026-07-28. This crate performs no
I/O, starts no tasks, and has no transport dependency. It classifies terminal
delivery, MRTR continuations, durable tasks, errors, cache delivery, quotas, and
spend caps.
Most applications should depend on the `mcp-usage-kit` facade. Use this crate
directly when you need only the deterministic protocol and pricing engine.