[PR #35] oracle.rs: CachedPrice raw 8-decimal, no normalization — profit math off by 10^10

[obchain]

PR: #35 (feat/10-chainlink-pricecache)
Commit: 5c36873
File: crates/charon-scanner/src/oracle.rs, CachedPrice struct and refresh()

Problem: CachedPrice.price stores raw latestRoundData() answer without decimal normalization. All five configured BSC feeds return 8-decimal values. CachedPrice carries .decimals field but provides no normalization method, and type-level rustdoc makes no mention of expected consumer scale.

Venus getUnderlyingPrice() returns prices scaled to 10^(36 - underlyingDecimals):

• 18-decimal tokens (WBNB, WETH): effective scale = 1e18 per USD
• 8-decimal tokens (BTCB): effective scale = 1e28 per USD

Profit calculator pairing Venus collateral value (WBNB balance in 1e18 units) with raw Chainlink BNB price (e.g., 3e11 for $300 in 8 decimals) without scaling computes USD value 10^10 times too small. Every liquidation appears unprofitable; bot skips all opportunities.

Naive scaling by 1e18 without accounting for 8-decimal feed overshoots by 1e10.

Risk: Silent mispricing — bot never executes (too small) or sizes incorrectly (too large). No runtime error, no log warning.

Fix:

impl CachedPrice {
    pub fn scaled_to(&self, target_decimals: u8) -> U256 {
        let diff = target_decimals.abs_diff(self.decimals);
        let factor = U256::from(10u64).pow(U256::from(diff));
        if target_decimals >= self.decimals {
            self.price * factor
        } else {
            self.price / factor
        }
    }
}

Document in struct rustdoc that price is raw feed-native decimals. Unit test scaled_to(18) on 8-decimal answer. Update all consumers to call scaled_to() with appropriate target before arithmetic.

[obchain]

Fixed on feat/10-chainlink-pricecache in 53b24e1.