DrainWeights::dust_threshold is below Bitcoin Core's dust limit for P2TR change

[noahjoeris]

Describe the bug

DrainWeights::dust_threshold computes the threshold from the drain's actual output_weight + spend_weight. For DrainWeights::TR_KEYSPEND that is 172 + 230 = 402 WU, which at the default 3 sat/vB dust relay feerate gives 302 sats. Bitcoin Core's dust rule for segwit outputs instead assumes a fixed 67 vB spend size, which is why a P2TR output is dust below 330 sats (matching the crate's own TR_DUST_RELAY_MIN_VALUE).

Since LowestFee::drain_value relies on dust_threshold, it can return a P2TR change output valued between 302 and 329 sats, which default Bitcoin Core relay policy treats as dust.

This issue was found by AI.

To Reproduce

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

#[test]
fn does_not_create_p2tr_change_below_core_dust_threshold() {
    let candidates = [Candidate {
        value: 1_000,
        weight: 1,
        input_count: 1,
        is_segwit: true,
    }];
    let mut cs = CoinSelector::new(&candidates);
    assert!(cs.select(0));

    let target = Target {
        fee: TargetFee::ZERO,
        outputs: TargetOutputs {
            value_sum: 1_000 - (TR_DUST_RELAY_MIN_VALUE - 10),
            weight_sum: 1,
            n_outputs: 1,
        },
        max_weight: None,
    };
    let mut metric = LowestFee {
        long_term_feerate: FeeRate::from_sat_per_vb(1.0),
        dust_relay_feerate: FeeRate::from_sat_per_vb(3.0),
        drain_weights: DrainWeights::TR_KEYSPEND,
    };

    // Fails: returns a drain with value 320.
    assert!(metric.drain(&cs, target).is_none());
}

Expected behavior

The dust threshold used by LowestFee should not produce change outputs that Bitcoin Core's default dust relay policy rejects (i.e. below 330 sats for P2TR at 3 sat/vB).