[flashloan] new public enums missing #[non_exhaustive] — future variant additions will be semver-breaking

[obchain]

Refs #39

File: crates/charon-core/src/types.rs (FlashLoanSource), crates/charon-core/src/flashloan.rs, crates/charon-flashloan/src/

Problem

FlashLoanSource in types.rs already lacks #[non_exhaustive] (flagged in PR #27 review, not yet fixed). PR #39 introduces FlashLoanQuote and likely additional enums in charon-flashloan. Any pub enum in a library crate that may gain variants in future milestones must carry #[non_exhaustive] from its first commit. Adding the attribute later is a semver-breaking change: existing exhaustive match arms become compile errors in downstream crates.

FlashLoanSource already has BalancerV2 and UniswapV3 variants with no adapters implemented — the design explicitly anticipates future variants. All match arms on it are currently exhaustive by accident.

Fix

Add #[non_exhaustive] to:

• FlashLoanSource in crates/charon-core/src/types.rs
• Any pub enum introduced by this PR in charon-flashloan/src/ or charon-core/src/flashloan.rs

If FlashLoanQuote is a struct, ensure all fields are pub so downstream crates can construct it, or provide a builder — #[non_exhaustive] on structs blocks struct-literal construction outside the crate.

[obchain]

Resolved in b9092e1 on feat/14-flashloan-router. #[non_exhaustive] added to the public FlashLoanSource enum in charon-core::types and to the new FlashLoanError enum in charon-core::flashloan. Future variants (BalancerV2 on Arbitrum, AaveV3 on other chains, AaveV2, Morpho, ...) can land without breaking downstream match arms.