Add Gen IV Great Marsh changing encounters

[Machine-Maker]

Change description

Closes #1496

• Adds new condition type for great marsh selection, x of 32 options.
• Adds encounters for all the changing pokemon, taking into account pre and post national dex states.

AI coding assistance disclosure

Claude Code helped me analyze decompiled diamond and platinum repos. Also generated the big huge changes based on how they work.

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.