fix(score): multiroom player skill calculation

[anthroplankton]

Description

Fix getMultiSkillRate form calculating "scale factor" to calculating "rate of change".

Related Issue

#403
#404

Motivation and Context

The scores in the multiplayer mode calculated by the "Song Recommender" website differ from those shown of the game.

The getMultiSkillRate function previously implemented the formula for "effective skill powers" based on the game's mechanics before the official change. The previous implementation:

sekai-viewer/src/utils/scoreCalc.ts

Lines 56 to 62 in 4301ecc

	const getMultiSkillRate = useCallback((skillRates: number[]) => {
	let multiSkillRate = 1 + skillRates[0];
	skillRates.forEach((v, i) => {
	if (i > 0) multiSkillRate *= 1 + v / 5;
	});
	return multiSkillRate - 1;
	}, []);

The old formula was:
$\left(1+ \mathtt{skill}_1\right) \cdot \left(1+ \frac{\mathtt{skill}_2}{5}\right) \cdot \left(1+ \frac{\mathtt{skill}_3}{5}\right) \cdot \left(1+ \frac{\mathtt{skill}_4}{5}\right) \cdot \left(1+ \frac{\mathtt{skill}_5}{5}\right) - 1$,
which represents a "rate of change".

The implementation in src/utils/scoreCalc.ts is based on representing an "effective skill power" as a "rate of change" .

Therefore, the updated formula should also represent a "rate of change," which is:
$\mathtt{skill}_1 + \frac{\mathtt{skill}_2}{5} + \frac{\mathtt{skill}_3}{5} + \frac{\mathtt{skill}_4}{5} + \frac{\mathtt{skill}_5}{5}$.

How Has This Been Tested?

• Chrome (Desktop)
• Chrome (Mobile)
• Firefox (Desktop)
• Firefox (Mobile)
• Safari (Desktop, optional)
• Safari (iPhone, optional)
• Safari (iPad, optional)

Screenshots (if appropriate):