[hieuk48]

🎯 Overview & Context

Resolves #277 by adding the SDK's reliability layer: retry classification, transaction recovery/lookup, simulation diagnostics, and structured transaction logging with automatic secret redaction.

Stacked on #284 (feat/sdk-contract-clients → #283 → #282 → #281). Retargets to main as the stack merges.

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📦 Key Features Added

1. RPC Retry Classification & Backoff (sdk/src/recovery/retry.ts)

• isRetryable(error): boolean: Distinguishes transient RPC/network/sequencing failures (503, rate limit, txBadSeq, ETIMEDOUT) from deterministic rejections (insufficient funds, unauthorized, validation).
• mayHaveReachedServer(error): boolean: Detects ambiguous timeouts (504 Gateway Timeout, ECONNRESET, fetch failed) where a transaction may have been accepted into the mempool.
• withRpcRetry(fn, options): Robust exponential backoff with full jitter and onRetry callback hooks.

2. Transaction Recovery (sdk/src/recovery/lookup.ts)

• lookupTransaction(hash, { server, timeoutMs, pollIntervalMs }): Recovers transaction outcome (pending | success | failed | timeout | not_found) without throwing raw RPC exceptions.
• Provides standard guidance: after an ambiguous failure, callers safely check status via lookupTransaction before blindly submitting again.

3. Simulation Diagnostics (sdk/src/diagnostics/simulation.ts & diagnostics.ts)

• summarizeSimulation(simulation): SimulationEstimate: Produces log-safe numeric summaries ({ instructions, readBytes, writeBytes, feeStroops }) while explicitly stripping raw XDR payloads to avoid leaking ledger arguments into telemetry.
• classifyContractError(err): ContractErrorDiagnostic | null: Maps raw Soroban host error asserts to normalized typed codes (POOL_PAUSED, NOT_A_MEMBER, DEADLINE_PASSED, etc.) adhering to existing repository taxonomy.

4. Structured Transaction Logging & Redaction (sdk/src/diagnostics/logger.ts)

• TxLogger interface & TxLogEvent covering all transaction phases: "build" | "simulate" | "sign" | "submit" | "confirm" | "error".
• redactSensitiveData() & createSafeLogEvent(): Strips Stellar secret keys (S...) and Bearer tokens by default.
• Reference consoleTxLogger implementation for development and testing.

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🧪 Verification & Acceptance Criteria

• All 14 test suites passed: 139/139 unit tests green (pnpm test)
• Strict TypeScript check passed (pnpm typecheck)
• Clean compilation output (pnpm build)
• Public API verification passed: 173 symbols exported (pnpm check:api)

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