Add compute budget billing guard

[modelsbridgeaicom-ship-it]

/claim #20

Summary

• adds a distinct AI compute budget-threshold and overage-notice guard under revenue-compute-budget-guard/
• evaluates synthetic compute usage against approved budgets, top-up balances, warning thresholds, hard-cap holds, notice freshness, and draft invoice amounts
• emits deterministic CLEAR_TO_BILL, SEND_NOTICE_BEFORE_BILLING, or HOLD_COMPUTE_BILLING decisions with reviewer-ready remediation evidence
• includes README requirement mapping plus generated JSON/Markdown demo artifacts

Validation

• node revenue-compute-budget-guard/test.js
• node revenue-compute-budget-guard/demo.js
• node --check revenue-compute-budget-guard/budgetGuard.js
• node --check revenue-compute-budget-guard/demo.js
• node --check revenue-compute-budget-guard/test.js
• git diff --check

Notes

• Synthetic sample data only
• No credentials, payment processors, external APIs, private customers, or live billing systems
• Scope is distinct from existing Revenue Infrastructure #20 slices such as trial/coupon pricing, commitment true-up, replay/idempotency, invoice delivery, tax/VAT, analytics-seat, and data-product freshness guards