Add payment webhook entitlement guard

[sravan27]

@algora-pbc /claim #20

Summary

• add a focused payment-webhook entitlement activation guard for Revenue Infrastructure
• block duplicate event IDs, stale/invalid signatures, test-mode activation, missing order references, amount/currency mismatch, out-of-order lifecycle events, double-credit grants, and refund/dispute reversals
• generate deterministic JSON/Markdown/SVG/PNG/MP4 finance proof artifacts from synthetic data

Validation

• node payment-webhook-entitlement-guard/test.js
• node payment-webhook-entitlement-guard/demo.js
• node --check payment-webhook-entitlement-guard/index.js
• node --check payment-webhook-entitlement-guard/sample-data.js
• node --check payment-webhook-entitlement-guard/test.js
• node --check payment-webhook-entitlement-guard/demo.js
• git diff --check
• ffprobe confirms payment-webhook-entitlement-guard/reports/demo.mp4 is H.264 1280x720, 12 seconds

Notes

• dependency-free core logic
• synthetic data only; no live payment credentials
• distinct from generic billing ledgers, usage metering ledgers, payment failover, disputes, SLA credits, royalties, forecasts, FX, invoice acceptance, sanctions/export control, consortium pricing, and AI compute idempotency meters