Implement cost.budget capability + BUDGET_EXHAUSTED error

[nficano]

ARCP v1.1 §9.6 introduces cost.budget as a runtime-enforced lease capability with counters. Not implemented.

Steps:

1. Add CostBudget capability type (max, unit, consumed) to lib/src/main/kotlin/dev/arcp/lease/
2. Implement runtime counter in ARCPRuntime — increments on metric events tagged as cost
3. When consumed >= max, raise BUDGET_EXHAUSTED error (see related error-code issue)
4. Subset semantics: child budget must be ≤ parent
5. Add samples/CostBudget/ mirroring typescript-sdk/examples/cost-budget/
6. Add tests

Reference: spec §9.6, §13.5.

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)