Feature/issue 520 523 split features

[D-Ochuko]

Summary

Adds four independent SDK primitives, each addressing one issue:

• Split payment rollback coordinator (src/splitRollbackCoordinator.ts): records a per-leg checkpoint for multi-recipient split payments, lets callers mark each leg's application-layer acknowledgement outcome, and exposes initiateRollback to identify incomplete legs and persist a rollback record via src/snapshot.ts. Wired into StellarSplitClient.submitPayment's waterfall path (getRollbackCoordinator()), which now opens a checkpoint and marks all submitted legs successful immediately after an atomic on-chain submission.
• Invoice payment progress tracker (src/paymentProgressTracker.ts): aggregates per-recipient payment status into a single InvoicePaymentProgress snapshot via subscribe(invoiceId, onProgress), polling each recipient leg independently with InvoiceStatusPoller and emitting invoiceProgressUpdated. Includes tranche progress when the invoice defines one (src/trancheProgress.ts).
• XLM-to-fiat price oracle adapter (src/priceOracle.ts): defines the PriceOracleAdapter contract (src/types.ts) and ships CoinGeckoPriceOracle, a default implementation backed by CoinGecko's /simple/price endpoint, with a new TTL-based RateCache (src/rateCache.ts). convertFiatToAsset in src/currencyConverter.ts uses a supplied oracle to convert fiat amounts to asset amounts. StellarSplitClientConfig accepts an optional priceOracle.
• Strict-send / strict-receive path query builder (src/pathQueryBuilder.ts): encapsulates Horizon path query assembly, validation, execution, and caching behind PathQueryBuilder. src/pathRouter.ts is refactored to delegate to it instead of duplicating query/caching logic inline.

Changes by commit

• Add RollbackCoordinator for split payment rollback reconciliation — closes Atomic Split Payment Rollback Coordinator #520
• Add PaymentProgressTracker for aggregated invoice payment progress — closes Invoice Payment Progress Tracker #521
• Add CoinGeckoPriceOracle default XLM-to-fiat price oracle adapter — closes XLM-to-Fiat Price Oracle Adapter #522
• Add PathQueryBuilder and refactor PathRouter to use it — closes Strict-Send and Strict-Receive Path Query Builder #523

Test plan

• npm run build
• npm test
• Manually exercise RollbackCoordinator.begin/markLegSuccess/markLegFailed/getIncomplete/initiateRollback against a simulated post-submission failure
• Manually exercise PaymentProgressTracker.subscribe/unsubscribe against mocked per-recipient status responses and confirm percentComplete increments as legs settle
• Manually exercise CoinGeckoPriceOracle.getPrice against mocked fetch responses, including a 429 (expect RateLimitError)
• Manually exercise PathQueryBuilder.forStrictSend/forStrictReceive/execute against mocked Horizon responses, including invalid-parameter cases (expect InvalidPathQueryError)

Note: this repo currently has pre-existing TypeScript/build issues unrelated to this change (see npx tsc --noEmit on main), so CI may not be fully green independent of this PR.

Closes #520
Closes #521
Closes #522
Closes #523

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