[flashloan] Router: cheapest-source selection (Aave V3 on BSC) + fallback chain

[obchain]

Context

The flash-loan source router picks the cheapest available source for each liquidation. BSC does not have Balancer V2 deployed — so Aave V3 (0.05% fee) is the primary choice. The router still speaks the full abstraction so multi-source selection just works once other chains ship.

Scope

• New crates/charon-flashloan/ crate
• FlashLoanRouter struct with pick_source(token, chain_id, amount) -> FlashLoanSource
• Per-chain source registry loaded from config ([flashloan.<name>])
• v0.1 logic: always returns FlashLoanSource::AaveV3 for BSC (only configured source)
• Shape scaffolding for future Balancer / Uniswap fallbacks (commented stubs)
• Returns Option<FlashLoanSource> (None = skip the liquidation — no source available)

Acceptance criteria

• charon-flashloan crate compiles independently
• pick_source returns Some(AaveV3) for BSC/any common token
• Returns None for an unconfigured chain
• Unit test covers the selection logic

References

• PRD section 5a (Flash Loan Router)
• PRD Diagram 05 (Flash Loan Source Selection)

[obchain]

Superseded — fee-priority router shipped in feat/14-flashloan-router.