Ledger Close Time Estimator

[Kingsman-99]

Description

Features like payment expiry guards and reminder schedulers need to estimate when a future ledger sequence number will close. Stellar targets a ~5-second ledger close interval, but actual intervals vary. Without a data-driven estimate, expiresAt and timebounds.maxTime calculations are imprecise. A ledger close time estimator should compute a rolling average close interval from recent ledger history and project future close times.

Technical Context

Add src/ledgerCloseEstimator.ts. Uses @stellar/stellar-sdk Server.ledgers() to fetch the last N ledger records and extracts closed_at timestamps. Computes a rolling-average close interval. Exposes estimateCloseTime(targetLedger: number): Date and estimateLedgerAtTime(targetTime: Date): number. Integrates with src/deadlineEngine.ts and src/txBuilder.ts for timebounds computation.

Acceptance Criteria

• LedgerCloseEstimator.calibrate(sampleSize = 20) fetches the last sampleSize ledgers from Horizon and computes the rolling-average close interval in milliseconds
• estimateCloseTime(targetLedger) returns a Date object representing the projected close time for a future ledger sequence
• estimateLedgerAtTime(targetTime) returns the projected ledger sequence that will be current at targetTime
• Re-calibrates automatically every calibrationIntervalMs (configurable, default 5 minutes)
• Unit tests mock ledger records with known timestamps and assert correct interval computation and projection
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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