Add cumulative variance logging engine with PriceVarianceEvent

[AugistineCreates]

This pr closes #286

• Introduces a new PriceVarianceEvent that records the percentage change
  (in basis points) between the new price and the previous baseline.
• set_price now captures the prior price, computes the variance via
  calculate_percentage_change_bps, emits the PriceVarianceEvent,
  and logs the variance event using the existing log_event helper.
• Ensures variance is logged even when the price does not change
  (variance = 0) to keep the on‑chain divergence index up‑to‑date.

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