Fix UnityObject reference serialization in Placeholders

[marko1olo]

🎯 What: Implemented serialization of UnityObject references inside the MapMagic.Nodes.Placeholders by storing origUniObj inside GenericPlaceholder and assigning it manually to SerObject inside the custom serialization hooks.
💡 Why: UnityObject references were not being serialized (as mentioned by the TODO), preventing proper saving of GenericPlaceholder nodes.
✅ Verification: Verified compilation and no regression by running full verify sweep.
✨ Result: Placeholders now correctly serialize UnityObject references.

---

PR created automatically by Jules for task 16878339498043222893 started by @marko1olo

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.