[lua] Fix elegy potency bug

[9001-Sols]

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• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Fixes Elegy potency gain from Song+ gear (ELEGY_EFFECT/ALL_SONGS_EFFECT) being 10x too low. gearBoost was multiplied by 6375/256 instead of 63750/256. Elegy power feeds HASTE_MAGIC, which is in /10000 units, so per-point gear potency is 25.5/1024 * 10000 ≈ 249.

Steps to test these changes

1. Cast Battlefield Elegy (base 2500 = 25% slow) with no Song+ gear; confirm ~25% slow
2. Equip ~10 points of Elegy + / All Songs + gear and recast
3. Confirm potency now scales up toward the 5000 (50% slow)

Test Results

Before	After

Before almost no perceivable effect, after matches BG wiki's +2 chart, as well as the original comment -- Simplified numbers of: 25.5 * 10000/1024:

[Xaver-DaRed]

power = power + math.floor(gearBoost * 25.6) * 10000 / 1024

[Xaver-DaRed]

This will match the values in this table exactly.