feat: add searchByMemo for local invoice arrays

[aigon-x]

Summary

Adds searchByMemo(invoices, query, opts?) to src/search.ts and exports it from src/index.ts — a substring search over Invoice.memo for an in-memory invoice array.

Payment memo text is a common way users tag invoices (e.g. "split:INV-001"), and src/search.ts could only search Horizon by status/recipient with no way to filter a local array by memo. This is a self-contained, synchronous helper that complements searchInvoices (which stays async/Horizon-backed).

Changes

• src/search.ts — new searchByMemo() + SearchByMemoOptions interface.
• src/index.ts — re-exports searchByMemo and SearchByMemoOptions.
• test/searchByMemo.test.ts — unit tests covering every acceptance criterion.

Acceptance criteria

• searchByMemo(invoices: Invoice[], query: string, opts?: { caseSensitive?: boolean }): Invoice[] exported from src/search.ts
• Returns invoices whose invoice.memo contains query as a substring; case-insensitive by default
• Empty query returns all invoices unchanged
• Invoices with memo of undefined / 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

Implementation notes

• The input array is never mutated (returns a new filtered array; the empty-query path returns the input as-is per the criterion).
• The helper is pure/synchronous — no Horizon dependency in the hot path.

How I tested

npx vitest run test/searchByMemo.test.ts
# ✓ test/searchByMemo.test.ts (7 tests) — all pass

Negative check (mandatory): with the fix reverted, all 7 tests fail; with the fix restored, all 7 pass.

Regression: npm test (client / retryPolicy / network-environment) — 110 tests pass.

tsc --noEmit introduces no new errors from the three files in this PR (the repo has unrelated pre-existing type errors elsewhere).

Fixes #614