Components.AI decorators and conversation actions

### Is your feature request related to a problem? Please describe the problem.

Components.AI has no reusable seam for per-message or per-turn decoration, preventing built-in and app-supplied actions from composing consistently.

### Describe the solution you'd like

Track API design and implementation of separate message and turn decorators, optional placement merging, accessible visibility and traversal, built-in action menus, copy/retry/edit actions, timestamps, and feedback.

This is a workstream under #68227. Its independently mergeable tasks are represented as native sub-issues.

### Additional context

Covers D4, D13, and F1-F10 from #68227.

Start with the parent design work in #68227 and review the native sub-issues linked from this workstream. Choose an independently mergeable task covering the listed decorator, action, accessibility, timestamp, or feedback areas; this issue itself does not identify a file, test, or single completion target.

Written by the indexing model from the issue text.