feat: add searchByMemo, validateSplitTotal/normalizeSplits, and has/purgeExpired to cache (Closes #614, #609, #606)

[waterWang]

Summary

This PR implements three independent features:

#614 — Add searchByMemo to search.ts

• searchByMemo(invoices, query, opts?) performs substring search on invoice memo fields
• Case-insensitive by default, with opts.caseSensitive override
• Returns all invoices unchanged when query is empty
• Skips invoices with undefined/null memo without error

#609 — Add validateSplitTotal and normalizeSplits to splitRatioValidator

• validateSplitTotal(splits, totalBasisPoints?) validates bigint splits sum to exact total (default 10000n)
• Throws StellarSplitError with code INVALID_RECIPIENT on mismatch
• normalizeSplits(amounts, total) distributes rounding remainder to the last element
• Pure bigint arithmetic — no floating-point conversion

#606 — Add has() and purgeExpired() to SimpleCache

• has(key) checks existence and TTL expiry
• purgeExpired() removes all expired entries in one sweep, returns count removed
• Respects disabled cache state (returns false/0 when disabled)

Tests

• test/search-memo.test.ts — 6 tests: substring match, case-insensitive default, case-sensitive override, empty query, null memo skip, no match
• test/splitRatioValidator.test.ts — 9 tests: exact sum passes, off-by-one throws, INVALID_RECIPIENT code, empty array throws, normalizeSplits round-trip, remainder distribution, over-sum handling
• test/cache.test.ts (extended) — 11 tests (8 new): has() exists, has() nonexistent, has() expired, has() disabled, purgeExpired() removes expired, purgeExpired() nothing expired, purgeExpired() disabled

All 26 tests pass.