Implement Post a Memo behavior with acceptance pipeline

Implements the post-memo behavior slice per specs/post-memo.feature:
- src/services/memo-post.js: compose, validate (empty/length), and broadcast
  a Memo post OP_RETURN (0x6d02) via the minimal-slp-wallet adapter surface.
- Stands up the Babashka APS acceptance pipeline (entrypoint generator,
  runtime, regex step handlers, convenience runner).
- Adds focused unit tests for the memo behavior.

By coder.

test/unit/memo-post.test.js

@@ -0,0 +1,133 @@
+/*
+  Unit tests for the Memo post behavior slice (src/services/memo-post.js).
+
+  These tests express the observable behavior described by
+  specs/post-memo.feature:
+    - a valid memo broadcasts an OP_RETURN transaction carrying the Memo post
+      prefix (0x6d02) and the message text, and the feed reflects the new post.
+    - an empty memo is rejected with a validation error and nothing is broadcast.
+    - an over-long memo is rejected with a length error and nothing is broadcast.
+*/
+
+'use strict'
+
+const test = require('node:test')
+const assert = require('node:assert/strict')
+
+const MemoPost = require('../../src/services/memo-post')
+
+// A fake wallet that records every broadcast attempt. It satisfies the small
+// adapter surface the MemoPost module needs: walletInfo, getUtxos(),
+// sendOpReturn().
+function fakeWallet (cashAddress = 'bitcoincash:qqlrzp23w08434twmvr4fxw672whkjy0py26r63g3d', utxos = [{ txid: 'utxo1' }]) {
+  const broadcasts = []
+  const wallet = {
+    walletInfo: { cashAddress },
+    getUtxos: async () => utxos,
+    sendOpReturn: async (walletInfo, bchUtxos, msg, prefix) => {
+      broadcasts.push({ walletInfo, bchUtxos, msg, prefix })
+      return 'fake-txid'
+    }
+  }
+  wallet.broadcasts = broadcasts
+  return wallet
+}
+
+// A fake feed that records posts added to the recent posts feed.
+function fakeFeed () {
+  const posts = []
+  return { posts, addPost: (p) => posts.push(p) }
+}
+
+test('MEMO_POST_PREFIX is the Memo post action 0x6d02', () => {
+  assert.equal(MemoPost.MEMO_POST_PREFIX, '6d02')
+})
+
+test('posting a valid memo broadcasts an OP_RETURN with the Memo post prefix and message', async () => {
+  const wallet = fakeWallet()
+  const feed = fakeFeed()
+  const memoPost = new MemoPost({ wallet, feed })
+
+  const txid = await memoPost.post('hello memo')
+
+  assert.equal(txid, 'fake-txid')
+  assert.equal(wallet.broadcasts.length, 1)
+  const b = wallet.broadcasts[0]
+  assert.equal(b.prefix, '6d02')
+  assert.equal(b.msg, 'hello memo')
+  // The broadcast uses the wallet's spendable UTXOs.
+  assert.equal(b.bchUtxos.length, 1)
+  // The wallet info passed to sendOpReturn is the authenticated wallet.
+  assert.equal(b.walletInfo.cashAddress, wallet.walletInfo.cashAddress)
+
+  // The feed reflects the new post from this address with this text.
+  assert.equal(feed.posts.length, 1)
+  assert.equal(feed.posts[0].text, 'hello memo')
+  assert.equal(feed.posts[0].address, wallet.walletInfo.cashAddress)
+})
+
+test('posting a memo at the maximum length (217) is accepted', async () => {
+  const wallet = fakeWallet()
+  const memoPost = new MemoPost({ wallet })
+
+  const msg = 'x'.repeat(217)
+  const txid = await memoPost.post(msg)
+  assert.equal(txid, 'fake-txid')
+  assert.equal(wallet.broadcasts[0].msg, msg)
+})
+
+test('posting an empty memo throws a validation error and broadcasts nothing', async () => {
+  const wallet = fakeWallet()
+  const feed = fakeFeed()
+  const memoPost = new MemoPost({ wallet, feed })
+
+  await assert.rejects(
+    memoPost.post(''),
+    (err) => err.code === 'memo_validation'
+  )
+  assert.equal(wallet.broadcasts.length, 0)
+  assert.equal(feed.posts.length, 0)
+})
+
+test('posting a whitespace-only memo throws a validation error and broadcasts nothing', async () => {
+  const wallet = fakeWallet()
+  const memoPost = new MemoPost({ wallet })
+
+  await assert.rejects(
+    memoPost.post('   '),
+    (err) => err.code === 'memo_validation'
+  )
+  assert.equal(wallet.broadcasts.length, 0)
+})
+
+test('posting a non-string memo throws a validation error', async () => {
+  const wallet = fakeWallet()
+  const memoPost = new MemoPost({ wallet })
+
+  await assert.rejects(
+    memoPost.post(42),
+    (err) => err.code === 'memo_validation'
+  )
+  assert.equal(wallet.broadcasts.length, 0)
+})
+
+test('posting an over-long memo (218) throws a length error and broadcasts nothing', async () => {
+  const wallet = fakeWallet()
+  const feed = fakeFeed()
+  const memoPost = new MemoPost({ wallet, feed })
+
+  await assert.rejects(
+    memoPost.post('y'.repeat(218)),
+    (err) => err.code === 'memo_length'
+  )
+  assert.equal(wallet.broadcasts.length, 0)
+  assert.equal(feed.posts.length, 0)
+})
+
+test('posting without a wallet reports a missing-wallet error', async () => {
+  const memoPost = new MemoPost({})
+  await assert.rejects(
+    memoPost.post('hello memo'),
+    (err) => /wallet/i.test(err.message)
+  )
+})