fix: Handle "relative" and "proportional" with encumbrance

[Coolthulhu]

Checklist

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• I wrote the PR title in conventional commit format.
• I ran the code formatter.
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Purpose of change

Fix #3861

Describe the solution

Use assign in a hacky way to have it handle the fields properly.

• Add tests that confirm it works

Describe alternatives you've considered

Revert the armor portions thing - it won't help us much, but it is certain to cause trouble until we rewrite the jsons to the new format.

Testing

• Spawn chitin gauntlets and biosilicified chitin gauntlets
• Compare encumbrance - biosilicified should have 15 encumbrance, while regular should have 10
• Load world with Magiclysm
• Spawn chitin gauntlets and demon chitin gauntlets
• Demon chitin gauntlets should have 9 encumbrance, while regular should have 10

Additional context