feat: add compressMetadata, estimateFeeForAmount, buildPaymentMemo helpers (Closes #619, #615, #610)

[waterWang]

Summary

Adds three independent SDK helpers:

#619 — compressMetadata / decompressMetadata (src/compression.ts)

• JSON + base64url round-trip with no padding
• compressMetadata throws StellarSplitError (CONTRACT_REJECTED) when encoded output exceeds maxBytes (default 512)
• decompressMetadata validates base64url charset, throws StellarSplitError (CONTRACT_REJECTED) on invalid input / JSON

#615 — estimateFeeForAmount (src/feeEstimator.ts)

• estimateFeeForAmount(amount, feeStats) → { feeLumens, feePercent, totalWithFee }
• feeLumens stays bigint; totalWithFee = amount + feeLumens; feePercent is 0 for zero amount
• Throws StellarSplitError (INVALID_RECIPIENT) for negative amount
• FeeStats type { baseFee, p50Fee, p99Fee } exported

#610 — buildPaymentMemo / parsePaymentMemo (src/memoBuilder.ts)

• Canonical split:{invoiceId} format, split:{invoiceId}:t{tranche} when tranche provided
• Truncated to Stellar's 28-byte text memo limit without cutting a multi-byte UTF-8 character
• parsePaymentMemo returns null for non-split memos, round-trips both formats

All helpers exported from src/index.ts.

Tests

• test/compressMetadata.test.ts — 10 tests (round-trip, oversized, invalid base64, empty object)
• test/estimateFeeForAmount.test.ts — 5 tests (fee calc, bigint, zero amount, negative throw)
• test/paymentMemo.test.ts — 8 tests (base/tranche format, truncation, round-trip, null)

23/23 tests passing.

Closes #619, #615, #610