Cached summary cards retain an author's old name after a rename

Problem/Motivation

Renaming a registered author does not invalidate cached summary-only comment cards. A fresh render array contains the new name, but the cached thread still displays the old name. The card reads the author name without adding the author entity as a cache dependency.

Steps to reproduce

1. Render and cache a discussion using summary-only density.
2. Rename a registered author whose comment is displayed.
3. Request the discussion again without manually clearing the render cache.
4. Observe the old name; the thread's cache tags omit the author's user tag.

Proposed resolution

Include cache dependencies for author data used by the card, while preserving field-access checks and cacheability.

Remaining tasks

• Add a warm-cache regression test that renames the author between requests.
• Verify the new name appears without a global cache clear.
• Cover summary-only rendering, where no rendered body supplies an incidental author dependency.

User interface changes

Cached bylines reflect author renames.

API changes

None anticipated beyond corrected cache metadata.

Data model changes

None anticipated.