Implement cost.budget + BUDGET_EXHAUSTED

[nficano]

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

Steps:

1. Add CostBudget lease capability with max, unit, consumed fields (use bcmath or Brick\Math for precision)
2. Runtime increments counter on cost-tagged metric events
3. On exceeding budget, emit BUDGET_EXHAUSTED error
4. Implement subset enforcement (child ≤ parent)
5. Add samples/cost_budget/ demo
6. Tests

Reference: spec §9.6.

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