Comment index built twice per node render

Problem/Motivation

build() loads the comment index for the thread, and the history lazy builder (CommentHistory::lazyBuilder) loads it again because the index is not passed through the lazy-builder arguments. Because the history builder is max-age 0 with user/user.permissions contexts, it runs on every node view, producing a second full 216-comment query plus access-check pass. The versions map also serializes and hashes every comment's toArray() on each of these runs.

Steps to reproduce

• Open a node with 200+ comments under ACT.
• Observe the comment index query and access checks run twice per page render.

Proposed resolution

• Thread the built index (or its ids + metadata) into the history lazy builder so it is not rebuilt.
• Alternatively, cache the index within the request so build() and lazyBuilder share it.

Remaining tasks

• Confirm the index is built once per node render.

User interface changes

None, beyond faster node render.

API changes

Change the lazy-builder argument list to accept the index or its precomputed data.

Data model changes

None anticipated.