feat: client modules, type definitions, performance tuning & error codes guides

[iam-mercy]

What this fixes

• Refactor client into modules #1079 Refactor client into modules: StellarSplitClient was a monolithic class containing thousands of lines across disparate domains (invoices, payments, disputes, admin actions, grouping, templates, and milestones), making maintenance, domain isolation, and modular consumption difficult.
• Add type definitions improvements #1080 Add type definitions improvements: The TypeScript type definitions lacked generic parameters for metadata and lifecycle statuses, functional result utilities, status narrowing type guards, and branded types to prevent type confusion between plain strings/bigints and domain types (stroops, addresses, invoice IDs).
• Create performance tuning guide #1078 Create performance tuning guide: Developers lacked documented guidance and empirical benchmarks for optimizing SDK performance across RPC connection pooling, request batching, multi-tier caching, circuit breaking, multi-endpoint load balancing, and Soroban storage TTL batching.
• Add error code reference guide #1077 Add error code reference guide: SDK error codes and panic mappings lacked a centralized reference guide documenting root causes, recovery strategies, retryability classifications, and concrete code examples.

What changed

1. Client Modularization (Refactor client into modules #1079):

   • Designed and created logical sub-modules under src/client/modules/:
     • InvoiceModule: Creation, cloning, updates, queries, cancellations, sync, snapshots, and verification.
     • PaymentModule: Payments, simulations, batch payments, receipt generation, refunds, and reconciliations.
     • DisputeModule: Dispute raising, arbiter voting, resolution, and evidence attachment.
     • AdminModule: Administrative freeze/unfreeze and timelocked actions.
     • GroupModule: Group creation, invoice attachment, release, and aggregation statistics.
     • TemplateModule: Template storage, template creation, and recurring invoice management.
     • MilestoneModule: Milestone completions, active milestone tracking, and funding round closure.
   • Attached module instances (invoices, payments, disputes, admin, groups, templates, milestones) to StellarSplitClient.
   • Maintained 100% backward compatibility for all existing methods on StellarSplitClient.
   • Re-exported modules and sub-clients in src/client/index.ts and src/index.ts.

2. Type Definitions & Utilities (Add type definitions improvements #1080):

   • Made Invoice<TMetadata, TStatus> and CreateInvoiceParams<TMetadata> generic with backward-compatible defaults.
   • Added Result<T, E> discriminated union with ok(), err(), isOk(), isErr(), unwrapResult(), mapResult(), mapErr(), and matchResult().
   • Added PaginatedResponse<T> generic interface and constructor helper.
   • Added branded types: StroopAmount, StellarAddress, InvoiceId with safe conversion helpers (toStroops, toStellarAddress, toInvoiceId).
   • Added status-narrowed invoice types (PendingInvoice, ReleasedInvoice, etc.) and type guard functions (isPendingInvoice, isReleasedInvoice, isInvoiceStatus).
   • Added recursive type utilities: DeepPartial, DeepReadonly, RequireKeys, OptionalKeys, StrictOmit, RequireAtLeastOne, RequireOnlyOne.

3. Performance Tuning Guide (Create performance tuning guide #1078):

   • Created docs/PERFORMANCE_TUNING.md documenting:
     • RPC connection pooling (rpcPoolSize) with 85% latency reduction benchmarks.
     • Request multiplexing & batching (setBatchingEnabled) with 93% network roundtrip reduction.
     • In-memory and optimistic caching (cache) with sub-millisecond reads.
     • Circuit breaker and adaptive throttling for outage protection.
     • Health-weighted multi-endpoint balancing (rpcEndpoints).
     • Soroban storage TTL batch bumps (bumpStorageTtlBatch) saving 62% in fees.
     • Real-time streaming (WebSocket/SSE) vs. Horizon polling with 97% lower lag.
     • Comprehensive performance decision matrix.

4. Error Code Reference Guide (Add error code reference guide #1077):

   • Created docs/ERROR_CODES.md cataloging the complete SDK error taxonomy across invoice lifecycle, payments, network/RPC, access control, disputes, milestones, and batch operations.
   • Added root causes, recovery actions, and TypeScript code examples for every error code.
   • Implemented missing error classes in src/errors.ts: RoundNotEndedError, WrongMilestoneError, NothingToClaimError, InvoiceExpiredError, PayerNotWhitelistedError, ContributionCapExceededError, PaymentCooldownActiveError, ContractFrozenError, InvoiceFullyFundedError, and DelegateLimitReachedError.
   • Updated isRetryable() to distinguish transient retryable conditions (RPC 429/503, timeouts, cooldowns) from terminal contract errors.

Testing

• Unit tests run via Vitest:
  • test/client.test.ts (68 passed)
  • test/retryPolicy.test.ts (31 passed)
  • test/network-environment.test.ts (11 passed)
  • test/clientModules.test.ts (12 passed)
  • test/typeImprovements.test.ts (16 passed)
  • test/errorCodes.test.ts (10 passed)
  • test/sdkExports.test.ts (3 passed)
  • Total: 7 test files, 151 tests passed, 0 failures.

Issues

closes #1080
closes #1079
closes #1078
closes #1077

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