fix(react): merge className with runtime-managed host classes

[ShaneK]

Resolves #485

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally for affected output targets
• Tests (npm test) were run locally and passed
• Prettier (npm run prettier) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

@lit/react maps React's className prop to the host's class attribute and rewrites it wholesale on every render. That wipes the classes the Stencil runtime manages on the host: hydrated, sc-* scope classes, mode classes, and state classes like interactive. So when an app drives className (for example a form library setting Ionic's ion-invalid / ion-touched validation classes), those app classes replace the runtime's classes instead of coexisting with them, and the component loses styling and hydration state.

Issue URL:

• bug: React output target v0.7.0 overwrites classNames #485
• bug: className updates on Ionic React form controls overwrite Stencil runtime classes (breaks class-based validation) ionic-team/ionic-framework#31237

What is the new behavior?

createComponent now wraps the @lit/react component and withholds className/class so React never writes the class attribute directly. It reconciles the app's class value against the live host in a layout effect using a new mergeClassNames helper. The merge keeps runtime-managed classes the app never set, appends new app classes, and removes only the app classes that were dropped since the previous render (tracked with a previousClassName ref). It uses an isomorphic layout effect so it falls back to useEffect during server rendering and doesn't warn.

Does this introduce a breaking change?

• Yes
• No

Other information

[OS-jacobbell]

Confirmed this fixes the issue and passes Ionic Framework's e2e tests.

[ShaneK]

@johnjenkins Would you have some time to review this one? Just for verification that it all makes sense