feat: add CoinSelector::select_srd (Single Random Draw)

[evanlinjin]

Motivation

Adds Single Random Draw coin selection — a port of Bitcoin Core's SelectCoinsSRD. Unlike run_bnb with LowestFee (which minimizes fees), SRD deliberately aims for a healthy-sized, privacy-friendly change output, avoiding tiny "toxic" change. It's the natural complement to the fee-minimizing metric from #49.

What it does

CoinSelector::select_srd adds candidates in random order until the change reaches change_lower, then returns the Drain to attach (or InsufficientFunds).

pub fn select_srd(
    &mut self,
    target: Target,
    drain_weights: DrainWeights,
    change_lower: u64,
    rng: impl FnMut() -> u64,
) -> Result<Drain, InsufficientFunds>

• Fixed lower bound, like Core. We stop as soon as the change reaches change_lower; the change amount comes out random on its own, because candidates are added in random order and the final change is wherever the last random input pushed it. Core uses the same fixed-threshold approach (and its comment notes SRD "will result in a random change amount anyway"), so there's no need to randomize the target.
• The stop condition reuses excess(target, drain_weights), so there's no new fee math — it inherits the existing rate/absolute/replacement handling.
• Zero new dependencies: RNG is a plain impl FnMut() -> u64 (the crate is deliberately dependency-free), used to shuffle the candidates.
• CHANGE_LOWER (Core's 50k) is added as the sensible default for change_lower.
• Already-selected candidates are kept and counted toward the target.

max_selection_weight handling (evicting the least-valuable inputs when over a weight cap) is left as a documented // TODO pending the max-weight PR.

Tests

tests/srd.rs covers: change ≥ change_lower and target met across 300 seeds; a custom change_lower is honored; insufficient funds; per-seed determinism; preselected inputs preserved. Full suite + no_std build + clippy are green.

🤖 Generated with Claude Code

[evanlinjin]

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