LowestFee score loses precision for fees above 2^24 sats due to f32 conversion

[noahjoeris]

Describe the bug

LowestFee::fee_score converts the exact integer long-term fee to f32 before wrapping it in Ordf32:

Ordf32((fee_for_the_tx as u64 + fee_for_spending_drain) as f32)

f32 only represents integers exactly up to 2^24 (~16.7M sats). Above that, distinct fee totals round to the same f32 and compare as equal. Since branch-and-bound only replaces the incumbent on a strictly smaller score, it can keep a selection that pays more sats in fees than another available selection.

This issue was found by AI.

To Reproduce

use bdk_coin_select::{
    metrics::LowestFee, Candidate, CoinSelector, DrainWeights, FeeRate, Target, TargetFee,
    TargetOutputs,
};

#[test]
fn bnb_distinguishes_large_fee_scores() {
    let exact_fee = 1u64 << 25;
    let candidates = [
        Candidate { value: exact_fee + 2, weight: 100, input_count: 1, is_segwit: false },
        Candidate { value: exact_fee, weight: 100, input_count: 1, is_segwit: false },
    ];
    let target = Target {
        outputs: TargetOutputs { value_sum: 0, weight_sum: 0, n_outputs: 0 },
        fee: TargetFee { absolute: exact_fee, ..TargetFee::ZERO },
        max_weight: None,
    };
    let metric = LowestFee {
        long_term_feerate: FeeRate::from_sat_per_wu(1.0),
        dust_relay_feerate: FeeRate::ZERO,
        drain_weights: DrainWeights::TR_KEYSPEND,
    };

    let mut cs = CoinSelector::new(&candidates);
    cs.run_bnb(target, metric, usize::MAX).expect("a valid selection exists");

    // Fails: selects the first candidate and pays `exact_fee + 2`.
    assert_eq!(cs.fee(target.value(), 0), exact_fee as i64);
}

Expected behavior

Selections whose fees differ by a whole number of sats should not compare as equal, so branch-and-bound picks the one with the lower fee.