Thread render cache: use a per-host tag instead of comment_list, and stop hard-coding the user context

Problem/Motivation

The thread render-cache entry inherits its cacheability from the comment index, with three effects:

• Signed-in markup is never shared. CommentIndexRepository::load() hard-codes the user context (src/CommentIndexRepository.php:192, and :173 for bounded empty results). The context was added for the in-request memo, whose key is already per account (:162-163), not because the index content varies per user. CommentHistory::lazyBuilder() adds user as well (src/Element/CommentHistory.php:49). This is documented in docs/loading-strategies.md Known limitations ("The signed-in thread render cache is per user").
• Any comment write invalidates every thread on the site. The index adds the global comment_list tag (src/CommentIndexRepository.php:193), which core invalidates on every comment save.
• Every cache hit validates one tag per comment. For readers without administer comments, core adds one comment:N tag per accessible comment. CommentThreadBuilder::build() merges $slice->cacheability (src/CommentThreadBuilder.php:213), so those tags reach the thread entry. The code comment at :225-231 says per-comment tags are not on it; that comment is wrong.

ADR 0009 phase 3, first bullet.

Proposed resolution

• Replace comment_list and the per-comment comment:N tags on the thread entry with act_comments:{entity_type}:{entity_id}, invalidated as in #1 (insert, update, delete, both hosts on a move). If this lands before #1, it introduces the tag and the hooks itself. Keep node:N, config and field tags. Per-comment entries in CommentSliceRenderer keep comment:N (src/CommentSliceRenderer.php:125-134).
• Stop adding user in load(). Keep the per-account memo key. Per-user parts of the slice (approve, edit and delete access, the relocated links) already bubble their own contexts.
• Fix the misleading comment at CommentThreadBuilder.php:225-231.
• Later, possibly in a follow-up: placeholder the per-user action links so that authenticated thread HTML varies only by permissions.

Remaining tasks

• Kernel tests:
  • a write on host A does not invalidate host B's thread;
  • delete and move invalidate the right hosts;
  • two signed-in users with the same roles share a thread entry, but not when an access result declares user;
  • the tag count on the thread entry is independent of the comment count.
• Audit every consumer of $index->cacheability() for reliance on user.

User interface changes

None.

API changes

New cache tag act_comments:{entity_type}:{entity_id}, documented for sites that need to invalidate it.

Data model changes

None.