Implement invoice clone SDK method with optimistic UI state, rollback, and clone chain resolver

[Kingsman-99]

Description

Build a full cloneInvoice SDK feature that includes: submitting the clone contract call, optimistic local state update, automatic rollback on failure, and a resolveCloneChain method that fetches the full lineage of cloned invoices.

Acceptance Criteria

• StellarSplitClient.cloneInvoice(sourceId: string, overrides: CloneOverrides): Promise<string> — submits the clone_invoice contract call and returns the new invoice ID
• CloneOverrides TypeScript type mirrors the contract struct: newDeadline?: number, newAmounts?: bigint[], newRecipients?: string[], newOverflowBehavior?: OverflowBehavior
• On submission failure, any optimistic cache entry written during the call must be rolled back
• resolveCloneChain(invoiceId: string): Promise<Invoice[]> — recursively fetches parent invoices via parent_invoice_id from get_invoice_ext until root is reached; returns array ordered root→leaf
• resolveCloneChain must handle cycles defensively (max depth 10, throws "clone chain depth exceeded")
• Both methods exported from src/index.ts with full TypeScript types
• cloneInvoice uses the existing retry logic from src/client.ts
• Tests cover: successful clone, failed clone with rollback verification, chain resolution with 3-deep lineage, cycle detection

Context

• Contract call pattern is in src/client.ts — follow pay() as reference for retry + submission
• src/types.ts needs CloneOverrides and updated InvoiceExt with parentInvoiceId and cloneDepth
• Cache rollback pattern should follow any existing cache invalidation in src/cache.ts

[dannyy2000]

I would like to work on this issue because I have experience building
TypeScript SDK clients that wrap smart contract calls, and exposing a cloneInvoice method follows a clear, well-defined pattern I can implement cleanly and efficiently. I can add the method to StellarSplitClient, wire up the contract call, export it properly, and cover both success and error cases in tests within a short turnaround