Add FX settlement invoice guard

[soave0529-oss]

/claim #20

Adds a self-contained fx-settlement-invoice-guard package for Revenue Infrastructure.

This slice validates whether multi-currency invoices can be released or closed when invoice currency, account currency, settlement currency, FX snapshot freshness, minor-unit rounding, processor settlement amounts, credit-note offsets, and material variance approvals are consistent.

Why this is distinct:

• not generic subscription billing
• not usage metering or compute replay
• not tax/VAT handling
• not quota rollover
• not grant milestone billing
• not committed-usage true-up
• not budget-threshold notices
• not analytics-seat licensing
• not data-product freshness
• not procurement approval
• not generic invoice delivery

Artifacts included:

• deterministic synthetic invoice packets
• JSON results
• Markdown reviewer report
• SVG summary
• short MP4 demo artifact

Validation run locally:

• npm test
• npm run demo
• node --check src/index.js
• node --check scripts/demo.js
• node --check test/fxSettlementInvoiceGuard.test.js
• git diff --check

Boundaries:

• synthetic invoice packets only
• no payment processor calls
• no live FX market data
• no bank details
• no credentials
• no private customer data

AI-assisted with OpenAI Codex; I reviewed the implementation and kept the module isolated.

[soave0529-oss]

Scope clarification after checking prior same-issue work: this PR is intentionally narrower than the earlier FX settlement reconciliation submission (#225). It focuses on invoice-release rounding controls and credit-offset safety: supported currency minor units, invoice/account/settlement currency consistency, stale quote holds, processor settlement delta tolerance, credit approval evidence, and material variance blocking. It does not attempt provider fee caps, remittance reference deduplication, cross-border tax evidence, booking reconciliation, or broader settlement ledger reconciliation.