Add unit tests for computePriceChange helper at positive, negative, and zero change scenarios

[Chucks1093]

Summary

The computePriceChange helper computes a signed percentage but has no unit tests covering all three outcome signs. Tests should explicitly verify positive, negative, and zero change and confirm the result is always rounded to two decimal places.

Scope

• Test oldest snapshot 100, latest 150 → returns 50.00
• Test oldest snapshot 200, latest 100 → returns -50.00
• Test oldest snapshot 100, latest 100 → returns 0.00
• Test a fractional result (oldest 3, latest 4) → returns 33.33 (rounded, not truncated)

Acceptance Criteria

• Positive change returns correct positive percentage
• Negative change returns correct negative percentage
• Zero change returns 0.00
• Fractional result rounded to two decimal places

ETA: 12 hours

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