Add invoice payment SLA tracker

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Creators offering invoices with implicit payment SLAs (e.g. "paid within 24h of creation") need a way to track compliance across their invoice history for reporting purposes.

Technical Context

New src/slaTracker.ts. Computes timeToFirstPayment and timeToFullFunding per invoice from Invoice.payments timestamps (requires ledger-time correlation, reuse src/timeMachine.ts's getInvoiceAtTime pattern).

Acceptance Criteria

• computeSlaReport(invoices: Invoice[], slaMs: number) returns { withinSla: number, breached: number, avgTimeToFund: number }
• Excludes invoices with zero payments from time-to-fund averages (NaN-safe)
• Exported from src/index.ts
• Tests: mixed compliant/breached set computes correct aggregate; empty input returns zeroed report

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[Padom2020]

Hi! I'd like to apply to work on this issue.
I'm a fullstack developer with over 5 years of experience, including TypeScript/SDK work and time-series/financial reporting logic. I've built highly complex production software including my ecommerce platform digemart.com, eventcrib.com, and stannscollegeddl.org.
My plan:

Create src/slaTracker.ts and implement computeSlaReport(invoices: Invoice[], slaMs: number), computing timeToFirstPayment and timeToFullFunding per invoice from Invoice.payments timestamps.
Reuse the ledger-time correlation pattern from timeMachine.ts's getInvoiceAtTime rather than re-deriving timestamp logic separately.
Exclude invoices with zero payments from time-to-fund averages, handling the empty-set case in a NaN-safe way (return 0, not NaN, for avgTimeToFund when no qualifying invoices exist).
Export computeSlaReport from src/index.ts.
Add tests: a mixed compliant/breached invoice set producing the correct aggregate counts and average, and an empty input returning a zeroed report ({ withinSla: 0, breached: 0, avgTimeToFund: 0 }).

I'll open a draft PR within 6 hours of confirming scope so maintainers can confirm the exact Invoice/payments shape and timeMachine.ts reuse approach.
Looking forward to contributing!