[shobhamerabacha-star]

Summary of Changes

Closes #43

This PR implements proper Stellar Soroban ScVal XDR event decoding and strict envelope validation for the OrbitPay analytics indexer, replacing loose object stringification and shallow spreading.

Key Implementations:

1. Soroban ScVal XDR Decoding (src/lib/indexer/decoder.ts):
   • Explicitly decodes xdr.ScVal instances, wrapped { xdr: string } responses, and base64 XDR strings using scValToNative.
   • Losslessly handles addresses (Address to public key string), 64-bit and 128-bit integers (u64, i128, u32, i32), symbols, maps, vectors, booleans, and timestamps.
2. Strict Event Envelope Validation:
   • Validates that eventId, ledger (> 0 integer), txHash, contractId, and non-empty topic are present and valid.
   • Treats missing or manufactured identities ("unknown", 0, empty strings) as visible decode failures rather than projecting corrupt entities.
3. Dead-Letter Quarantining & Metrics Integration (src/lib/indexer/engine.ts):
   • Captures visible decode failures, increments decodeFailures.inc({ contract }), and records them in prisma.deadLetterEvent table.
   • Enforces KNOWN_TOPICS per configured contract (treasury, payroll, vesting, governance) and quarantines unknown topics with QuarantineError.
4. Comprehensive Test Suite (src/__tests__/decoder.test.ts):
   • Unit tests covering envelope failures, ScVal primitive and struct decoding.
   • Real captured SDK-shaped fixtures for Treasury (TreasuryDeposit), Payroll (StreamCreated), and Governance (VoteCast).