feat: implement issues #212-#215 (reputation, idempotency, payload gu…

[cybermax4200]

Closes #212 - Invoice Creator Reputation Aggregator

• New `src/reputation.ts` with `computeCreatorReputation()`
• Computes completion rate, average funding time, dispute rate, and overall score
• Configurable weights for each component

Closes #213 - Payload Size Guard

• New `src/payloadGuard.ts` with `validateInvoicePayload()`
• Pre-flight check for oversized invoice metadata
• Configurable limits for recipients, memo length, and serialized size
• Hooked into `StellarSplitClient.createInvoice()` via `payloadGuard` config

Closes #214 - Idempotency Key Manager

• New `src/idempotency.ts` with `IdempotencyManager` class
• SHA-256 based deterministic keys from source address + operation XDR
• Configurable TTL and max entries
• Hooked into `StellarSplitClient._submitTx()` to short-circuit duplicate submissions via `idempotency` config

Closes #215 - Invoice Payment Forecast Model

• New `src/forecast.ts` with `computePaymentForecast()`
• Blends current invoice prediction with historical data from similar invoices (same creator, similar amount range)
• Configurable amount tolerance and minimum sample size
• Extends existing `computePrediction()` from predictor.ts

All features include full test coverage and are exported from the public API."

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