feat: implement decodeMemo() transaction helper — closes #111

[spiffamani]

Summary

Implements decodeMemo() helper for decoding Stellar memo objects, closing #111.

Changes

• src/transactions/builder.ts — Added decodeMemo() and DecodedMemo type
• src/transactions/index.ts — Re-exports decodeMemo and DecodedMemo
• src/index.ts — Exports decodeMemo and DecodedMemo from public surface
• tests/unit/transactions/decodeMemo.test.ts — 7 unit tests

Memo Type Handling

Memo Type	Return Value
MEMO_NONE	null
MEMO_TEXT	original string
MEMO_ID	numeric value as string
MEMO_HASH	hex digest string
MEMO_RETURN	hex digest string

Usage

import { decodeMemo } from '@petad/stellar-sdk';
import { Memo } from '@stellar/stellar-sdk';

decodeMemo(Memo.none());        // { type: 'MEMO_NONE', value: null }
decodeMemo(Memo.text('hello')); // { type: 'MEMO_TEXT', value: 'hello' }
decodeMemo(Memo.id('12345'));   // { type: 'MEMO_ID', value: '12345' }

Test Results

Tests: 85 passed, 85 total
Test Suites: 10 passed, 10 total

Checklist

• All memo types handled
• Exported from src/transactions/builder.ts
• Available through public index
• npm test passes
• No new runtime dependencies

[amina69]

@spiffamani you are making pr to the wrong branch, read the conributing.md