Refactor topic feed: reduce CRAP, DRY, and add property coverage

- Add topics controller error-handling unit tests to bring handleError
  CRAP from 14.7 to 4.0.
- Consolidate duplicate getTopicPosts/getPostsByAddr HTTP page fetches
  into a shared getPage helper in memo-db (DRY).
- Add TopicQuery property tests: listTopics conservation/ordering,
  getTopicPostTxids newest-first pagination, and room/txid key round-trips.
- Add client topic-services property tests for getTopic/getPost lookup
  completeness.

By refactorer.

psf-memo-client/src/services/memo-db.js

@@ -52,17 +52,8 @@ class MemoDb {
     return this.getRecent('/topics', 'getTopics', {})
   }
 
-  async getTopicPosts (room, { limit = 100, offset = 0 } = {}) {
-    try {
-      const result = await this.axios.get(
-        `${config.backend}/topics/${encodeURIComponent(room)}/posts`,
-        { params: { limit, offset } }
-      )
-      return result.data
-    } catch (err) {
-      console.error('Error in getTopicPosts()')
-      throw err
-    }
+  async getTopicPosts (room, opts = {}) {
+    return this.getPage(`/topics/${encodeURIComponent(room)}/posts`, 'getTopicPosts', opts)
   }
 
   // GET a paginated 'recent' listing endpoint.
@@ -79,6 +70,20 @@ class MemoDb {
     }
   }
 
+  // GET a paginated resource page at a full path.
+  async getPage (path, name, { limit = 100, offset = 0 } = {}) {
+    try {
+      const result = await this.axios.get(`${config.backend}${path}`, {
+        params: { limit, offset }
+      })
+
+      return result.data
+    } catch (err) {
+      console.error(`Error in ${name}()`)
+      throw err
+    }
+  }
+
   // GET a level endpoint that resolves an address. Returns null on 404.
   async getLevelResource (endpoint, addr, name) {
     try {
@@ -95,18 +100,8 @@ class MemoDb {
     }
   }
 
-  async getPostsByAddr (addr, { limit = 100, offset = 0 } = {}) {
-    try {
-      const result = await this.axios.get(
-        `${config.backend}/posts/by/${encodeURIComponent(addr)}`,
-        { params: { limit, offset } }
-      )
-
-      return result.data
-    } catch (err) {
-      console.error('Error in getPostsByAddr()')
-      throw err
-    }
+  async getPostsByAddr (addr, opts = {}) {
+    return this.getPage(`/posts/by/${encodeURIComponent(addr)}`, 'getPostsByAddr', opts)
   }
 
   async getPostThread (txid) {

psf-memo-client/test/property/topic-services.property.test.js

@@ -0,0 +1,80 @@
+/*
+  Property tests for the topic discovery and topic feed page controllers.
+
+  The unit tests probe getTopic / getPost at a few fixed fixtures. These
+  properties pin down the lookup invariants over broad random inputs:
+
+    - TopicDiscoveryPage.getTopic finds any topic that was loaded.
+    - TopicFeedPage.getPost finds any post that was loaded.
+    - Lookups are stable: a loaded item is always found, and an unknown key
+      returns null.
+*/
+
+'use strict'
+
+const test = require('node:test')
+const { seededRandom, forAll, intGen } = require('./harness')
+const TopicDiscoveryPage = require('../../src/services/topic-discovery-page')
+const TopicFeedPage = require('../../src/services/topic-feed-page')
+
+const rng = seededRandom(20260828)
+
+function randomRoom () {
+  return 'room-' + Math.floor(rng() * 1e6)
+}
+
+function randomTxid () {
+  return 't' + rng().toString(36).slice(2, 10) + Math.floor(rng() * 1e6)
+}
+
+test('TopicDiscoveryPage.getTopic finds any loaded topic', async () => {
+  await forAll(
+    (i) => {
+      const n = intGen(rng, 0, 20)()
+      const topics = []
+      for (let j = 0; j < n; j++) {
+        topics.push({ room: randomRoom(), postCount: intGen(rng, 0, 1000)() })
+      }
+      return topics
+    },
+    async (topics) => {
+      const memoDb = { async getTopics () { return { topics } } }
+      const page = new TopicDiscoveryPage({ memoDb })
+      await page.load()
+
+      for (const topic of topics) {
+        const found = page.getTopic(topic.room)
+        if (!found || found.room !== topic.room) return false
+      }
+      if (page.getTopic('does-not-exist') !== null) return false
+      return true
+    },
+    { label: 'topic discovery lookup completeness' }
+  )
+})
+
+test('TopicFeedPage.getPost finds any loaded post', async () => {
+  await forAll(
+    (i) => {
+      const n = intGen(rng, 0, 20)()
+      const posts = []
+      for (let j = 0; j < n; j++) {
+        posts.push({ txid: randomTxid(), text: 'post ' + j })
+      }
+      return posts
+    },
+    async (posts) => {
+      const memoDb = { async getTopicPosts () { return { posts, pagination: { total: posts.length } } } }
+      const page = new TopicFeedPage({ memoDb, room: 'bitcoin' })
+      await page.load()
+
+      for (const post of posts) {
+        const found = page.getPost(post.txid)
+        if (!found || found.txid !== post.txid) return false
+      }
+      if (page.getPost('does-not-exist') !== null) return false
+      return true
+    },
+    { label: 'topic feed lookup completeness' }
+  )
+})