[Bug]: sp-menu: should skip hidden items when navigating via keyboard

### Code of conduct

- [x] I agree to follow this project's code of conduct.

### Impacted component(s)

sp-menu, sp-menu-item

### Library version

1.9.0

### Expected behavior

Hidden sp-menu-items would not participate in focus traversal of sp-menu items.

### Actual behavior

Hidden sp-menu-items are tab stops on focus traversal of sp-menu items.

### Screenshots

### What browsers are you seeing the problem in?

Chrome

### How can we reproduce this issue?

1. Open https://opensource.adobe.com/spectrum-web-components/components/menu/#anatomy in Chrome
2. Inspect an sp-menu-item in Chrome Dev Tools
3. Add a hidden attribute
4. Use the up/down arrows to navigate the list
5. Note the tabstop on the hidden item

### Sample code or abstract reproduction which illustrates the problem

_No response_

### Severity

None

### Logs taken while reproducing problem

_No response_

### Would you like to track this issue in Jira?

- [x] Yes, please tell me the ticket number!

Start with the sp-menu and sp-menu-item entry points and reproduce the issue in the menu anatomy example using Chrome. Trace keyboard focus traversal, then verify that items with the hidden attribute are skipped while visible menu items remain reachable with the up/down arrows.

Written by the indexing model from the issue text.