[flashloan] FlashLoanRouter sort lacks available_liquidity tiebreaker for equal-fee providers

[obchain]

Refs #39

File: crates/charon-flashloan/src/router.rs — sort logic

Problem

The router sorts providers by fee_rate_bps ascending. When two providers share the same fee, Rust's stable sort preserves insertion order, which is determined by registration order — not by operational preference. When two equal-fee providers are available, the one with greater available_liquidity is strictly better: more liquidity reduces the probability of the flash loan failing mid-execution.

At v0.1 with a single Aave adapter this is dormant. The router is designed for multi-provider use (BalancerV2 and UniswapV3 variants exist in FlashLoanSource) and this ordering bug is latent from day one.

Fix

providers.sort_by(|a, b| {
    a.fee_rate_bps
        .cmp(&b.fee_rate_bps)
        .then_with(|| b.available_liquidity.cmp(&a.available_liquidity))
});

[obchain]

Resolved in b9092e1 on feat/14-flashloan-router. New FlashLoanRouter::with_liquidity_tiebreaker async constructor probes each provider's available_liquidity(token) and sorts fee_rate_millionths ASC, then available_liquidity DESC (a.fee.cmp(&b.fee).then_with(|| b.liq.cmp(&a.liq))). The plain FlashLoanRouter::new keeps the fee-only ordering for call sites that don't want the probe cost. Covered by liquidity_tiebreaker_prefers_deeper_pool_at_same_fee.