[core] LendingProtocol async trait

[obchain]

Context

The main protocol-adapter boundary. Scanner and executor consume this trait without caring which lending protocol sits behind it — that's how we'll add Aave / Compound / Morpho later without touching the scanning or execution logic.

Scope

• LendingProtocol trait in charon-core/src/traits.rs
• fn id() -> ProtocolId
• async fn fetch_positions(borrowers: &[Address]) -> Result<Vec<Position>>
• fn get_liquidation_params(position: &Position) -> Result<LiquidationParams>
• fn build_liquidation_calldata(params: &LiquidationParams) -> Result<Vec<u8>>
• Uses #[async_trait] so the trait is dyn-compatible for Box<dyn LendingProtocol> registries

Acceptance criteria

• Trait defined in charon-core/src/traits.rs
• All four methods present with correct signatures
• Trait is Send + Sync-bounded for tokio multi-threading
• Crate compiles with async-trait dependency

References

• Delivered in commit 07bf907
• PRD section 4a (Lending Protocol Trait)

[obchain]

Superseded — LendingProtocol async trait shipped in feat/03-lending-protocol-trait.