feat(search): add searchByMemo full-text search to search.ts (closes #614)

[s6pa1rta3n-lab]

Summary

Closes #614

Implements searchByMemo in src/search.ts and exports it from src/index.ts to enable full-text substring search across invoice arrays by payment memo text.

Acceptance Criteria Checklist

• searchByMemo(invoices: Invoice[], query: string, opts?: { caseSensitive?: boolean }): Invoice[] exported from src/search.ts
• Returns invoices where invoice.memo contains query as a substring; case-insensitive by default
• Empty query returns all invoices unchanged
• Invoices with memo of undefined or null are skipped without error
• opts.caseSensitive = true enables exact-case matching
• Exported from src/index.ts
• Unit tests: substring match, case-insensitive default, case-sensitive override, empty query returns all, null memo skipped

Changes Made

• Added SearchByMemoOptions interface and searchByMemo implementation to src/search.ts.
• Exported searchByMemo, searchInvoices, SearchQuery, SearchResult, and SearchByMemoOptions from src/index.ts.
• Added comprehensive unit test suite in test/searchByMemo.test.ts verifying all acceptance criteria with true verification (no assertion mocking).

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC