[Feature] Revisit event signature - decide Action<> or EventHandler<T>

### Problem

title says it all

### Proposed solution

think through WinUI alignment, ease of coding, AI efficiency... most likely (unless perf is a concern) EventHandler probably makes sense

### Alternatives considered

_No response_

### Additional context

_No response_

### Confirmation

- [x] I have searched [existing issues and specs](https://github.com/microsoft/microsoft-ui-reactor/issues?q=is%3Aissue) for prior discussion of this idea.

No files, tests, or entry points are named. Start by locating the current event declarations, then review the WinUI event-signature conventions and compare the coding and performance considerations mentioned in the issue; done means reaching and recording a project-level decision between Action<> and EventHandler.

Written by the indexing model from the issue text.