[scanner] Chainlink PriceCache: latestRoundData ABI + staleness check + per-asset config

[obchain]

Context

Health-factor computation needs USD prices. Chainlink is the primary source; per BSC asset we configure the feed address, then poll latestRoundData each block. Stale feeds are rejected (threshold 10 min) to avoid valuing positions against dead oracles.

Scope

• charon-scanner/src/price.rs with a PriceCache struct
• sol! macro bindings for AggregatorV3Interface.latestRoundData()
• Per-asset config: [chainlink.<chain>.<asset>] feed_address = "0x..."
• Top-5 BSC assets for v0.1: BNB, USDT, USDC, BTC, ETH
• Stale price check: reject if updatedAt < now() - 10 min (configurable)
• Cache stores { price: U256, decimals: u8, updated_at: u64 }
• Updates alongside the block listener

Acceptance criteria

• charon-scanner compiles with Chainlink ABI bindings
• PriceCache::refresh() fetches all configured feeds in one multicall
• Stale prices are dropped with a WARN log naming the asset + age
• PriceCache::get(&asset) -> Option<Price> returns None if stale / missing
• Config supports adding more assets by editing the TOML only

References

• PRD section 4c (Price Engine)
• Chainlink BSC feeds: https://data.chain.link/bsc/mainnet

[obchain]

Superseded — Chainlink PriceCache shipped in feat/10-chainlink-pricecache.