Add thread query performance specification

Bound /posts/:txid/thread to per-thread reads: prefix-scan postChildren
by parent txid and scope like counts to the thread's txids instead of
the global postLikes scan. Record both implementation parts in the
feature backlog.

By specifier.

psf-memo-db/specs/thread-query-performance.feature

@@ -0,0 +1,27 @@
+# Scenarios: Thread query performance - 1
+#
+# GET /posts/:txid/thread currently does work proportional to the whole
+# database, not to the requested thread:
+#   - get-post-thread.js calls buildLikeCountMap(), which iterates every
+#     postLikes entry and loads one post per like just to compute counts.
+#   - get-post-thread.js loadChildTxids() iterates every postChildren entry
+#     for each node in the thread instead of prefix-scanning that node's
+#     parent key.
+# This feature pins the endpoint to bounded, per-thread reads. Thread shape,
+# reply ordering, and per-node likeCount remain specified by like-counts.feature.
+Feature: Thread query performance
+
+  Background:
+    Given a psf-memo-db instance with posts, postHeights, addrPostHeights, postChildren, likes, and postLikes stores
+    Given the fixture "posts-with-likes" is loaded into the posts and likes stores
+
+  Scenario Outline: Thread query performance - 1 GET /posts/:txid/thread performs work bounded by the thread
+    When the client requests the thread for <txid>
+    Then the post with txid <txid> has likeCount <likeCount>
+    And the postChildren store was read at most <max_entries> entries
+    And no more than <limit> posts are loaded by txid
+
+    Examples:
+      | txid       | likeCount | max_entries | limit |
+      | post-200-a | 2         | 1           | 2     |
+      | post-200-b | 1         | 0           | 1     |

specs/feature-backlog.md

@@ -157,16 +157,36 @@ Reference: https://memo.sv/protocol (Wayback snapshot 2025-12-15)
 
 ---
 
-## Next up: account avatar display — DONE (2026-09-05)
+## Next up: Thread query performance (psf-memo-db)
+
+`GET /posts/:txid/thread` is slow because `get-post-thread.js` does work
+proportional to the whole database instead of the requested thread. Two
+independent parts, one feature spec:
+
+- **Part A — like counts scoped to the thread.** `execute()` calls
+  `buildLikeCountMap()`, which iterates every `postLikes` entry and loads one
+  post per like. Build the thread first, then count likes for the thread's
+  txids via the existing `countLikesForTxids`.
+- **Part B — child loading uses the parent index.** `loadChildTxids()`
+  iterates every `postChildren` entry once per thread node. Prefix-scan
+  `parentTxid:childTxid` (as `countRepliesForTxids` already does) instead.
+
+Spec: `psf-memo-db/specs/thread-query-performance.feature`. Affected
+component: D (`psf-memo-db`). No client, indexer, or protocol changes.
+
+Thread shape, reply ordering, and per-node `likeCount` remain specified by
+`psf-memo-db/specs/like-counts.feature` (scenario 3). `buildLikeCountMap`
+still has unit/property coverage; the coder should update it if the method is
+removed.
+
+---
+
+## Previous: account avatar display — DONE (2026-09-05)
 
 The `/account` page now renders the avatar image when an avatar URL is set
 (pure `AvatarImage` component + `AccountPage` display helpers). Merged to
 `master` at `5afaa64`.
 
-Next feature: TBD — current direction is front-end improvements to
-`psf-memo-client` (UI/UX polish, accessibility, performance, responsiveness,
-state handling, error surfacing).
-
 ## Notes for future cycles
 
 - Broadcast result (txid) is returned immediately; the action appears in the