Render real attributed strings; support + operator on Text

[dfabulich]

• Depends on Real attributed strings skip-foundation#122

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device
• REQUIRED: I have checked whether this change requires a corresponding update in the Skip Fuse UI repository (link related PR if applicable)
  Not done
• OPTIONAL: I have added an example of any UI changes to the Showcase sample app
  Set attributes on AttributedStrings, add them together, & add Texts skipapp-showcase#104

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Like skiptools/skip-foundation#122 this PR is a draft because it was fully generated by Cursor; I've barely reviewed it and tested it.

In particular, I haven't tested it at all in Skip Fuse, due to skiptools/skipapp-showcase#103