[protocols] charon-protocols crate + Venus adapter (getAccountLiquidity + vToken bindings)

[obchain]

Context

First concrete implementation of LendingProtocol. Venus doesn't expose a direct health factor — we compute it from collateral × collateral factor / debt, using the Comptroller and vToken ABIs. Also the proving ground for the charon-protocols crate that future adapters (Aave, Compound, Morpho) will live in.

Scope

• Create crates/charon-protocols/ crate and add to workspace members
• sol! macro bindings for Venus Comptroller (getAccountLiquidity, getAssetsIn, markets) and VToken (borrowBalanceCurrent, balanceOfUnderlying, underlying)
• VenusAdapter struct implementing LendingProtocol
• fetch_positions(borrowers) — multicall to query position state in one RPC round
• Compute health factor from collateral value × collateral factor / debt value
• Seed a small hardcoded list of known BSC borrowers for initial testing
• Subscribe to Venus Borrow events to accumulate the borrower set over time

Acceptance criteria

• charon-protocols crate compiles independently
• VenusAdapter::new(config) connects to BSC and can query the Comptroller
• fetch_positions(&[borrower]) returns a populated Position with non-zero collateral/debt for a known borrower
• Position mapping produces a sensible health_factor (> 0, ≤ ~10 for real accounts)
• Event subscription for Borrow events runs without errors

References

• PRD section 4b (Protocol-Specific Adapters)
• Venus docs: https://docs-v4.venus.io/

[obchain]

Superseded — Venus adapter shipped in feat/08-venus-adapter.