Add grant compute budget guard

[shaiananvari8]

Summary

• add a grant-backed AI compute budget guard for SCIBASE revenue infrastructure
• block billing when grant award ids are missing, grants are expired, sponsor holds exist, institutional invoices lack purchase orders, budgets are exceeded, or compute margins fall below policy
• include deterministic reviewer artifacts: JSON packet, Markdown report, and SVG summary

Verification

• npm test from grant-compute-budget-guard
• npm run demo from grant-compute-budget-guard

Demo evidence

• grant-compute-budget-guard/reports/compute-budget-report.md
• grant-compute-budget-guard/reports/compute-budget-packet.json
• grant-compute-budget-guard/reports/summary.svg

/claim #20