Add Ultra Burst trigger and make Ultra Necrozma battle-only

[MiquelRForgeFlow]

Change description

Introduce a new ultra-burst-item form trigger and mark Ultra Necrozma as battle-only, triggered by holding Ultranecrozium Z (like Mega/Primal held-item transformations).

Is distinct from held-item trigger to make it easy to customize, like making it trigger from Dusk and Dawn forms instead of default form.

AI coding assistance disclosure

I only asked AI to add the new ultra-burst-item. I defined it manually in the command line, and AI added it in the code.

Contributor check list

• I have written a description of the contribution and explained its motivation.
• I have written tests for my code changes (if applicable).
• I have read and understood the AI Assisted Contribution guidelines.
• I will own this change in production, and I am prepared to fix any bugs caused by my code change.

[MiquelRForgeFlow]

@Naramsim